Ameresco, Inc. Annual report pursuant to Section 13 and 15(d)

Business Segment Information - Revenue (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue from External Customer [Line Items]
Revenues	$ 217,371	$ 205,375	$ 196,982	$ 167,410	$ 211,133	$ 204,744	$ 166,665	$ 134,610	$ 787,138	$ 717,152	$ 651,227
Project
Revenue from External Customer [Line Items]
Revenues									545,053	506,550	454,200
Energy Assets
Revenue from External Customer [Line Items]
Revenues									95,776	69,241	64,882
Operations and Maintenance
Revenue from External Customer [Line Items]
Revenues									65,236	60,574	63,082
Integrated-PV
Revenue from External Customer [Line Items]
Revenues									41,349	38,796	29,325
Other services
Revenue from External Customer [Line Items]
Revenues									$ 39,724	$ 41,991	$ 39,738