Ameresco, Inc. Quarterly report pursuant to Section 13 or 15(d)

Equity and Earnings Per Share - Additional Information (Details) - USD ($)				9 Months Ended
	Mar. 15, 2021	Mar. 15, 2021	Mar. 09, 2021	Sep. 30, 2021	Sep. 30, 2020
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Proceeds from equity offering, net of offering costs				$ 120,084,000	$ 0
Proceeds used to repay revolving credit facility				$ 80,000,000
Class A Common Stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Sale of stock shares issued in transaction (in shares)			2,500,000
Offering price (in dollar per share)			$ 44.00
Proceeds from equity offering, net of offering costs			$ 104,326,000
Offering costs			$ 5,674,000
Class A Common Stock \| Over-Allotment Option
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Sale of stock shares issued in transaction (in shares)	375,000
Proceeds from equity offering, net of offering costs	$ 15,758,000
Offering costs	$ 742,000
Class A Common Stock \| Selling Shareholders
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Sale of stock shares issued in transaction (in shares)		805,000
Offering price (in dollar per share)	$ 44.00	$ 44.00
Proceeds from equity offering, net of offering costs		$ 0