| Income Taxes (Tables)
 | 12 Months Ended | 
| Dec. 31, 2015 | 
|---|
| Income Tax Disclosure [Abstract] |  | 
| Domestic and Foreign Income Taxes | 
The components of income before income taxes are as follows: 
 
|  |  
|  |  |  |  |  |  |  |  |  |  |  |  |  
|   | Year Ended December 31, |  
|   | 2015 |   | 2014 |   | 2013 |  
| Domestic | $ | 17,860 | 
 |   | $ | 14,505 | 
 |   | $ | 7,705 | 
 |  
| Foreign | (17,568 | ) |   | (8,213 | ) |   | (4,946 | ) |  
| Income before provision for income taxes | $ | 292 | 
 |   | $ | 6,292 | 
 |   | $ | 2,759 | 
 |  | 
| Income Tax Provision (Benefit) | 
The components of the provision (benefit) for income taxes are as follows: 
 
|  |  
|  |  |  |  |  |  |  |  |  |  |  |  |  
|   | Year Ended December 31, |  
|   | 2015 |   | 2014 |   | 2013 |  
| Current: |   | 
 |   |   | 
 |   |   | 
 |  
|    Federal | $ | 224 | 
 |   | $ | (2,659 | ) |   | $ | 10,114 | 
 |  
|    State | 603 | 
 |   | 1,826 | 
 |   | 3,499 | 
 |  
|    Foreign | 443 | 
 |   | (814 | ) |   | 371 | 
 |  
|   | 1,270 | 
 |   | (1,647 | ) |   | 13,984 | 
 |  
| Deferred: |   | 
 |   |   | 
 |   |   | 
 |  
|    Federal | 1,728 | 
 |   | (3,263 | ) |   | (10,315 | ) |  
|    State | 134 | 
 |   | 574 | 
 |   | (2,099 | ) |  
|    Foreign | (289 | ) |   | 245 | 
 |   | (1,225 | ) |  
|   | 1,573 | 
 |   | (2,444 | ) |   | (13,639 | ) |  
|   | $ | 2,843 | 
 |   | $ | (4,091 | ) |   | $ | 345 | 
 |  | 
| Deferred Income Tax Assets and Liabilities | 
Deferred tax assets and liabilities consist of the following: 
|  |  
|  |  |  |  |  |  |  |  |  
|   | December 31, |  
|   | 2015 |   | 2014 |  
| Deferred tax assets: |   | 
 |   |   | 
 |  
| Compensation accruals | $ | 3,330 | 
 |   | $ | 2,774 | 
 |  
| Reserves | 3,651 | 
 |   | 3,638 | 
 |  
| Other accruals | 2,252 | 
 |   | 2,893 | 
 |  
| Net operating losses | 8,220 | 
 |   | 7,498 | 
 |  
| Interest rate swaps | 1,804 | 
 |   | 1,715 | 
 |  
| Energy efficiency | 18,110 | 
 |   | 19,116 | 
 |  
| Deferred revenue | 1,479 | 
 |   | 1,769 | 
 |  
| Gross deferred income tax assets | 38,846 | 
 |   | 39,403 | 
 |  
| Valuation allowance | (7,122 | ) |   | (3,995 | ) |  
| Total deferred income tax assets | $ | 31,724 | 
 |   | $ | 35,408 | 
 |  
| Deferred tax liabilities: |   | 
 |   |   | 
 |  
| Depreciation | $ | (32,542 | ) |   | $ | (31,326 | ) |  
| Contract refinancing | (304 | ) |   | (437 | ) |  
| Canada | (2,234 | ) |   | (5,659 | ) |  
| United Kingdom | (538 | ) |   | (396 | ) |  
| Acquisition accounting | (116 | ) |   | (159 | ) |  
| Total deferred income tax liabilities | (35,734 | ) |   | (37,977 | ) |  
| Deferred income tax liabilities, net | $ | (4,010 | ) |   | $ | (2,569 | ) |  | 
| Reconciliation of Effective Tax Rates | 
The following is a reconciliation of the effective tax rates: 
|  |  
|  |  |  |  |  |  |  |  |  |  |  |  |  
|   | Year Ended December 31, |  
|   | 2015 |   | 2014 |   | 2013 |  
| Income before income tax | $ | 292 | 
 |   | $ | 6,292 | 
 |   | $ | 2,759 | 
 |  
| Federal statutory tax expense | $ | 102 | 
 |   | $ | 2,202 | 
 |   | $ | 966 | 
 |  
| State income taxes, net of Federal benefit | 604 | 
 |   | 666 | 
 |   | 201 | 
 |  
| Net state impact of deferred rate change | 55 | 
 |   | 264 | 
 |   | (69 | ) |  
| Non deductible expenses | 933 | 
 |   | 764 | 
 |   | 818 | 
 |  
| Impact of reserve for uncertain tax positions | (1,772 | ) |   | (977 | ) |   | 1,190 | 
 |  
| Stock-based compensation expense | 402 | 
 |   | 415 | 
 |   | 373 | 
 |  
| Energy efficiency preferences | (3,280 | ) |   | (9,517 | ) |   | (3,280 | ) |  
| Foreign items and rate differential | 1,556 | 
 |   | 719 | 
 |   | 349 | 
 |  
| Valuation allowance | 4,255 | 
 |   | 1,408 | 
 |   | (276 | ) |  
| Miscellaneous | (12 | ) |   | (35 | ) |   | 73 | 
 |  
|   | $ | 2,843 | 
 |   | $ | (4,091 | ) |   | $ | 345 | 
 |  
| Effective tax rate: |   |   |   | 
 |   |   | 
 |  
| Federal statutory rate expense | 35.0 |  % |   | 35.0 |  % |   | 35.0 |  % |  
| State income taxes, net of Federal benefit | 206.8 |  % |   | 10.6 |  % |   | 7.3 |  % |  
| Net state impact of deferred rate change | 18.8 |  % |   | 4.2 |  % |   | (2.5 | )% |  
| Non deductible expenses | 319.5 |  % |   | 12.1 |  % |   | 29.6 |  % |  
| Impact of reserve for uncertain tax positions | (606.8 | )% |   | (15.5 | )% |   | 43.1 |  % |  
| Stock-based compensation expense | 137.7 |  % |   | 6.6 |  % |   | 13.5 |  % |  
| Energy efficiency preferences | (1,123.3 | )% |   | (151.3 | )% |   | (118.9 | )% |  
| Foreign items and rate differential | 532.9 |  % |   | 11.4 |  % |   | 12.6 |  % |  
| Valuation allowance | 1,457.2 |  % |   | 22.4 |  % |   | (10.0 | )% |  
| Miscellaneous | (4.1 | )% |   | (0.6 | )% |   | 2.6 |  % |  
|   | 973.7 |  % |   | (65.1 | )% |   | 12.3 |  % |  | 
| Unrecognized Tax Benefits | 
A reconciliation of the beginning and ending balances of the total amounts of gross unrecognized tax benefits is as follows: 
|  |  
|  |  |  |  |  |  |  |  |  
|   | Year Ended December 31, |  
|   | 2015 |   | 2014 |  
| Balance, beginning of year | $ | 3,700 | 
 |   | $ | 9,200 | 
 |  
| Additions for prior year tax positions | 200 | 
 |   | 1,700 | 
 |  
| Settlements paid to tax authorities | — | 
 |   | — | 
 |  
| Reductions of prior year tax positions | (1,700 | ) |   | (7,200 | ) |  
| Balance, end of year | $ | 2,200 | 
 |   | $ | 3,700 | 
 |  |