Consolidated Statements of Shareholders' Equity - USD ($) $ in Thousands	Common Stock [Member]	Additional Paid-In Capital [Member]	Retained Earnings (Deficit) [Member] Cumulative Effect, Period of Adoption, Adjustment [Member]	Retained Earnings (Deficit) [Member]	Accumulated other comprehensive income (loss) [Member]	Cumulative Effect, Period of Adoption, Adjustment [Member]	Total
Balance at beginning of period at Dec. 31, 2017	$ 843	$ 1,265,043		$ (612,840)	$ 0		$ 653,046
Balance (in shares) at Dec. 31, 2017	84,302,000
Net income				1,324,487			1,324,487
Other comprehensive income							0
Issuance of common stock under employee benefit plans, net of forfeitures and shares withheld to cover taxes		14,173					14,173
Issuance of common stock under employee benefit plans, net of forfeitures and shares withheld to cover taxes, shares	58,000
Net issuance of common stock upon exercise of stock options	$ 8	57,160					57,168
Net issuance of common stock upon exercise of stock options, shares	745,000
Share-based compensation		18,806					18,806
Share repurchases, including fees	$ (61)	(93,119)		(1,620,833)			(1,714,013)
Share repurchases, including fees, shares	(6,061,000)
Balance at end of period (Adoption of ASU 2016-02 [Member]) at Dec. 31, 2018			$ (1,410)			$ (1,410)
Balance at end of period at Dec. 31, 2018	$ 790	1,262,063		(909,186)	0		353,667
Balance (in shares) at Dec. 31, 2018	79,044,000
Net income				1,391,042			1,391,042
Other comprehensive income					4,890		4,890
Issuance of common stock under employee benefit plans, net of forfeitures and shares withheld to cover taxes		15,302					15,302
Issuance of common stock under employee benefit plans, net of forfeitures and shares withheld to cover taxes, shares	46,000
Net issuance of common stock upon exercise of stock options	$ 5	46,101					46,106
Net issuance of common stock upon exercise of stock options, shares	406,000
Share-based compensation		20,534					20,534
Share repurchases, including fees	$ (39)	(63,240)		(1,369,512)			$ (1,432,791)
Share repurchases, including fees, shares	(3,877,000)						(3,877,000)
Balance at end of period at Dec. 31, 2019	$ 756	1,280,760		(889,066)	4,890		$ 397,340
Balance (in shares) at Dec. 31, 2019	75,619,000						75,618,659
Net income				1,752,302			$ 1,752,302
Other comprehensive income					(7,045)		(7,045)
Issuance of common stock under employee benefit plans, net of forfeitures and shares withheld to cover taxes		17,314					17,314
Issuance of common stock under employee benefit plans, net of forfeitures and shares withheld to cover taxes, shares	48,000
Net issuance of common stock upon exercise of stock options	$ 3	46,279					46,282
Net issuance of common stock upon exercise of stock options, shares	288,000
Share-based compensation		21,259					21,259
Share repurchases, including fees	$ (48)	(84,771)		(2,002,375)			$ (2,087,194)
Share repurchases, including fees, shares	(4,832,000)						(4,832,000)
Balance at end of period at Dec. 31, 2020	$ 711	$ 1,280,841		$ (1,139,139)	$ (2,155)		$ 140,258
Balance (in shares) at Dec. 31, 2020	71,123,000						71,123,109