Annual report pursuant to Section 13 and 15(d)

Business Acquisitions and Related Transactions - Narrative (Details)

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Business Acquisitions and Related Transactions - Narrative (Details) (USD $)
12 Months Ended 1 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Jul. 31, 2012
FAME
Apr. 30, 2011
Quantum
Aug. 31, 2010
Quantum
Sep. 30, 2012
Quantum
Sep. 30, 2012
AEG
Jul. 31, 2011
AEG
Aug. 31, 2011
Ameresco Southwest
Dec. 31, 2011
AIS
Business Acquisition [Line Items]                      
Initial cash payment           $ 6,150,000          
Additional payment 0 1,956,366 0   1,956,366            
Cash paid       4,486,950         11,993,236 50,057,113 4,497,141
Security deposit with third-party escrow agent       900,000         1,000,000   900,000
Maximum additional consideration for former stockholders of acquiree             865,000 5,000,000 5,000,000    
Fair value of additional consideration for former stockholders of acquiree                 1,652,000    
Valuation accrual                 $ 1,000,000