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Summary of Significant Accounting Policies - Goodwill and Intangible Assets (Details) (USD $)
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3 Months Ended | 12 Months Ended | 3 Months Ended | 3 Months Ended | |||||||||||
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Sep. 30, 2012
Agreement
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Dec. 31, 2012
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Dec. 31, 2011
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Dec. 31, 2010
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Sep. 30, 2012
APS
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Sep. 30, 2012
FAME
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Jul. 31, 2012
FAME
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Jul. 31, 2011
FAME
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Sep. 30, 2012
AEG
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Aug. 31, 2011
AEG
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Dec. 31, 2011
Ameresco Southwest
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Jul. 31, 2012
AIS
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Dec. 31, 2007
Interest Rate Swap
contract
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Mar. 31, 2010
Interest Rate Swap
Interest Rate Swap Contract 3
Swap
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Jul. 31, 2011
Interest Rate Swap
Interest Rate Swap Contract 4
Swap
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| Goodwill [Line Items] | |||||||||||||||
| Number of instruments | 2 | 1 | 1 | ||||||||||||
| Impairment of goodwill | $ 1,016,325 | $ 0 | $ 0 | ||||||||||||
| Percentage of stock acquired | 100.00% | 100.00% | 100.00% | ||||||||||||
| Number of purchase agreements | 2 | ||||||||||||||
| Goodwill recorded for acquisitions | 1,886,945 | 1,886,945 | 8,728,169 | 16,545,434 | 1,549,467 | ||||||||||
| Additional payment | $ 0 | $ 1,956,366 | $ 0 | ||||||||||||
| X | ||||||||||
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- Definition
Capital Stock Acquired, Percentage No definition available.
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| X | ||||||||||
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- Definition
Number of Purchase Agreements No definition available.
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| X | ||||||||||
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- Definition
Amount of goodwill arising from a business combination, which is the excess of the cost of the acquired entity over the amounts assigned to assets acquired and liabilities assumed. Reference 1: http://www.xbrl.org/2003/role/presentationRef
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| X | ||||||||||
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- Definition
The number of derivative instruments of a particular group held by the entity. Reference 1: http://www.xbrl.org/2003/role/presentationRef
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| X | ||||||||||
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- Definition
Loss recognized during the period that results from the write-down of goodwill after comparing the implied fair value of reporting unit goodwill with the carrying amount of that goodwill. Goodwill is assessed at least annually for impairment. Reference 1: http://www.xbrl.org/2003/role/presentationRef
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| X | ||||||||||
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- Details
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| X | ||||||||||
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- Definition
Cash outflow representing an adjustment to the purchase price of a previous acquisition. No definition available.
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