Financial Instruments - Restricted Cash (Details) - USD ($) $ in Thousands		Sep. 30, 2021	Dec. 31, 2020	Dec. 31, 2019
Variable Interest Entity [Line Items]
Restricted cash, current	[1]	$ 65,315	$ 52,470
Restricted cash, non-current	[1]	132,725	117,293
Restricted cash, total		198,040	169,763
Consolidated Entity, Excluding VIEs
Variable Interest Entity [Line Items]
Restricted cash, current		64,059	26,706
Restricted cash, non-current		117,501	286
PPA Entities
Variable Interest Entity [Line Items]
Restricted cash, current		1,256	4,698
Restricted cash, non-current		15,224	15,320
PPA Entities | PPA Company 2
Variable Interest Entity [Line Items]
Restricted cash, current		34,200	20,300
Restricted cash, non-current		65,200	88,400
Restricted cash, total		99,400
PPA Entities | PPA Company 3b
Variable Interest Entity [Line Items]
Restricted cash, current		1,200	700
Restricted cash, non-current		13,300	13,300
Restricted cash, total				$ 20,000
PPA Entities | Power Purchase Agreements Entities
Variable Interest Entity [Line Items]
Restricted cash, current		1,256	25,764
Restricted cash, non-current		$ 15,224	$ 117,007

[1] We have variable interest entities, which represent a portion of the consolidated balances recorded within these financial statement line items in the condensed consolidated balance sheets (see Note 11 - Portfolio Financings).