CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Cumulative Effect, Period of Adoption, Adjusted Balance	Class A	Class A Cumulative Effect, Period of Adoption, Adjusted Balance	Class B Convertible	Class B Convertible Cumulative Effect, Period of Adoption, Adjusted Balance	Additional Paid-in Capital	Additional Paid-in Capital Cumulative Effect, Period of Adoption, Adjusted Balance	Treasury Stock	Treasury Stock Cumulative Effect, Period of Adoption, Adjusted Balance	Accumulated Other Comprehensive Income (Loss)	Accumulated Other Comprehensive Income (Loss) Cumulative Effect, Period of Adoption, Adjusted Balance	Retained Earnings	Retained Earnings Cumulative Effect, Period of Adoption, Adjusted Balance
Beginning Balance at Dec. 31, 2019	$ 508,559		$ 16		$ 2		$ 593,583		$ (658,880)		$ (9,651)		$ 583,489
Beginning Balance (in shares) at Dec. 31, 2019			15,888,000		2,035,000				(7,807,000)
Net (loss) income	657		$ 0		$ 0		0		$ 0		0		657
Other comprehensive income (loss)	(547)		0		0		0		0		(547)		0
Issuance of class A common stock under stock option plans	340		$ 0		$ 0		340		$ 0		0		0
Issuance of class A common stock under stock option plans (in shares)			3,000		0				0
Purchases of treasury stock	(50,747)		$ 0		$ 0		0		$ (50,747)		0		0
Purchases of treasury stock (in shares)			0		0				(355,000)
Share-based compensation expense	3,164		$ 0		$ 0		3,164		$ 0		0		0
Ending Balance at Mar. 31, 2020	461,426		$ 16		$ 2		597,087		$ (709,627)		(10,198)		584,146
Ending Balance (in shares) at Mar. 31, 2020			15,891,000		2,035,000				(8,162,000)
Beginning Balance at Dec. 31, 2019	508,559		$ 16		$ 2		593,583		$ (658,880)		(9,651)		583,489
Beginning Balance (in shares) at Dec. 31, 2019			15,888,000		2,035,000				(7,807,000)
Net (loss) income	(10,185)
Other comprehensive income (loss)	2,751
Ending Balance at Sep. 30, 2020	389,292		$ 16		$ 2		604,974		$ (782,104)		(6,900)		573,304
Ending Balance (in shares) at Sep. 30, 2020			15,937,000		2,014,000				(8,684,000)
Beginning Balance at Mar. 31, 2020	461,426		$ 16		$ 2		597,087		$ (709,627)		(10,198)		584,146
Beginning Balance (in shares) at Mar. 31, 2020			15,891,000		2,035,000				(8,162,000)
Net (loss) income	3,387		$ 0		$ 0		0		$ 0		0		3,387
Other comprehensive income (loss)	1,011		0		0		0		0		1,011		0
Issuance of class A common stock under stock option plans	1,562		$ 0		$ 0		1,562		$ 0		0		0
Issuance of class A common stock under stock option plans (in shares)			12,000		0				0
Purchases of treasury stock	(11,128)		$ 0		$ 0		0		$ (11,128)		0		0
Purchases of treasury stock (in shares)			0		0				(90,000)
Share-based compensation expense	2,202		$ 0		$ 0		2,202		$ 0		0		0
Ending Balance at Jun. 30, 2020	458,460		$ 16		$ 2		600,851		$ (720,755)		(9,187)		587,533
Ending Balance (in shares) at Jun. 30, 2020			15,903,000		2,035,000				(8,252,000)
Net (loss) income	(14,229)		$ 0		$ 0		0		$ 0		0		(14,229)
Other comprehensive income (loss)	2,287		0		0		0		0		2,287		0
Conversion of class B to class A common stock	0		$ 0		$ 0		0		0		0		0
Conversion of class B to class A common stock (in shares)			21,000		(21,000)
Issuance of class A common stock under stock option plans	1,714		$ 0		$ 0		1,714		$ 0		0		0
Issuance of class A common stock under stock option plans (in shares)			13,000		0				0
Purchases of treasury stock	(61,349)		$ 0		$ 0		0		$ (61,349)		0		0
Purchases of treasury stock (in shares)			0		0				(432,000)
Share-based compensation expense	2,409		$ 0		$ 0		2,409		$ 0		0		0
Ending Balance at Sep. 30, 2020	389,292		$ 16		$ 2		604,974		$ (782,104)		(6,900)		573,304
Ending Balance (in shares) at Sep. 30, 2020			15,937,000		2,014,000				(8,684,000)
Net (loss) income	2,661		$ 0		$ 0		0		$ 0		0		2,661
Other comprehensive income (loss)	3,015		0		0		0		0		3,015		0
Conversion of class B to class A common stock	0		$ 0		$ 0		0		0		0		0
Conversion of class B to class A common stock (in shares)			50,000		(50,000)
Issuance of class A common stock under stock option plans	47,466		$ 0		$ 0		47,466		$ 0		0		0
Issuance of class A common stock under stock option plans (in shares)			320,000		0				0
Share-based compensation expense	2,801		$ 0		$ 0		2,801		$ 0		0		0
Equity component of convertible senior notes, net of issuance cost and deferred tax liability	107,810		0		0		107,810		0		0		0
Ending Balance at Dec. 31, 2020	$ 553,045	$ (106,853)	$ 16	$ 0	$ 2	$ 0	763,051	$ (107,810)	$ (782,104)	$ 0	(3,885)	$ 0	575,965	$ 957
Ending Balance (in shares) at Dec. 31, 2020			16,307,000		1,964,000				(8,684,000)
Accounting Standards Update Description	ASU 2020-06
Net (loss) income	$ (110,020)		$ 0		$ 0		0		$ 0		0		(110,020)
Other comprehensive income (loss)	(1,836)		0		0		0		0		(1,836)		0
Issuance of class A common stock under stock option plans	23,854		$ 0		$ 0		23,854		$ 0		0		0
Issuance of class A common stock under stock option plans (in shares)			159,000		0				0
Share-based compensation expense	6,848		$ 0		$ 0		6,848		$ 0		0		0
Ending Balance at Mar. 31, 2021	365,038		$ 16		$ 2		685,943		$ (782,104)		(5,721)		466,902
Ending Balance (in shares) at Mar. 31, 2021			16,466,000		1,964,000				(8,684,000)
Beginning Balance at Dec. 31, 2020	553,045	$ (106,853)	$ 16	$ 0	$ 2	$ 0	763,051	$ (107,810)	$ (782,104)	$ 0	(3,885)	$ 0	575,965	$ 957
Beginning Balance (in shares) at Dec. 31, 2020			16,307,000		1,964,000				(8,684,000)
Net (loss) income	(445,503)
Other comprehensive income (loss)	(2,896)
Ending Balance at Sep. 30, 2021	461,314		$ 17		$ 2		1,118,761		$ (782,104)		(6,781)		131,419
Ending Balance (in shares) at Sep. 30, 2021			17,078,000		1,964,000				(8,684,000)
Beginning Balance at Mar. 31, 2021	365,038		$ 16		$ 2		685,943		$ (782,104)		(5,721)		466,902
Beginning Balance (in shares) at Mar. 31, 2021			16,466,000		1,964,000				(8,684,000)
Net (loss) income	(299,347)		$ 0		$ 0		0		$ 0		0		(299,347)
Other comprehensive income (loss)	397		0		0		0		0		397		0
Issuance of class A common stock under stock option plans	244		$ 0		0		244		0		0		0
Issuance of class A common stock under stock option plans (in shares)			2
Share-based compensation expense	11,087		$ 0		0		11,087		0		0		0
Ending Balance at Jun. 30, 2021	77,419		$ 16		$ 2		697,274		$ (782,104)		(5,324)		167,555
Ending Balance (in shares) at Jun. 30, 2021			16,468,000		1,964,000				(8,684,000)
Net (loss) income	(36,136)		$ 0		$ 0		0		$ 0		0		(36,136)
Other comprehensive income (loss)	(1,457)		0		0		0		0		(1,457)		0
Issuance of class A common stock under stock option plans	7,282		$ 0		$ 0		7,282		$ 0		0		0
Issuance of class A common stock under stock option plans (in shares)			50,000		0				0
Issuance of class A common stock under employee stock purchase plans	2,854		$ 0		$ 0		2,854		$ 0		0		0
Issuance of class A common stock under employee stock purchase plans, (in shares)			5,000		0
Share-based compensation expense	11,883		$ 0		$ 0		11,883		0		0		0
Issuance of class A common stock under public offerings	399,469		$ 1		$ 0		399,468		0		0		0
Issuance of class A common stock under public offerings, (in shares)			555,000		0
Ending Balance at Sep. 30, 2021	$ 461,314		$ 17		$ 2		$ 1,118,761		$ (782,104)		$ (6,781)		$ 131,419
Ending Balance (in shares) at Sep. 30, 2021			17,078,000		1,964,000				(8,684,000)