CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Additional Paid-In Capital	Accumulated Deficit	Convertible Preferred Stock	Common Stock Class A	Common Stock Class B	Common Stock Class C	Common Stock Class D	Total
BALANCE at Dec. 31, 2019	$ 979,834	$ (796,806)	$ 0	$ 2	$ 3	$ 3	$ 39	$ 183,075
Consolidated net loss	0	(21,767)	0	0	0	0	0	(21,767)
Repurchase of shares of common stock	(3,598)	0	0	0	0	0	(3)	(3,601)
Exercise of options for common stock	1,974	0	0	0	0	0	2	1,976
Adjustment of redeemable noncontrolling interests to estimated redemption value	(884)	0	0	0	0	0	0	(884)
Stock-based compensation expense	661	0	0	0	0	0	0	661
BALANCE at Jun. 30, 2020	977,987	(818,573)	0	2	3	3	38	159,460
BALANCE at Dec. 31, 2019	979,834	(796,806)	0	2	3	3	39	183,075
BALANCE at Dec. 31, 2020	991,769	(804,919)	0	4	3	3	38	186,898
Consolidated net loss	0	17,873	0	0	0	0	0	17,873
Repurchase of shares of common stock	(904)	0	0	0	0	0	(1)	(905)
Issuance of shares of Class A common stock	33,278	0	0	4	0	0	0	33,282
Exercise of options for common stock	315	0	0	0	0	0	0	315
Adjustment of redeemable noncontrolling interests to estimated redemption value	(1,425)	0	0	0	0	0	0	(1,425)
Stock-based compensation expense	425	0	0	0	0	0	0	425
BALANCE at Jun. 30, 2021	$ 1,023,458	$ (787,046)	$ 0	$ 8	$ 3	$ 3	$ 37	$ 236,463