Condensed 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, 2018	$ 790	$ 1,262,063	$ (1,410)	$ (909,186)	$ 0	$ (1,410)	$ 353,667
Balance (in shares) at Dec. 31, 2018	79,044
Net income				1,066,126			1,066,126
Total other comprehensive income (loss)							0
Issuance of common stock under employee benefit plans, net of forfeitures and shares withheld to cover taxes		11,576					11,576
Issuance of common stock under employee benefit plans, net of forfeitures and shares withheld to cover taxes, shares	36
Net issuance of common stock upon exercise of stock options	$ 3	28,700					28,703
Net issuance of common stock upon exercise of stock options, shares	233
Share-based compensation		15,540					15,540
Share repurchases, including fees	$ (36)	(58,335)		(1,249,612)			$ (1,307,983)
Share repurchases, including fees, shares	(3,585)						(3,585,000)
Balance at end of period at Sep. 30, 2019	$ 757	1,259,544		(1,094,082)	0		$ 166,219
Balance (in shares) at Sep. 30, 2019	75,728
Balance at beginning of period at Jun. 30, 2019	$ 767	1,258,930		(1,115,015)	0		144,682
Balance (in shares) at Jun. 30, 2019	76,690
Net income				391,293			391,293
Total other comprehensive income (loss)							0
Issuance of common stock under employee benefit plans, net of forfeitures and shares withheld to cover taxes		3,523					3,523
Issuance of common stock under employee benefit plans, net of forfeitures and shares withheld to cover taxes, shares	11
Net issuance of common stock upon exercise of stock options	$ 1	8,774					8,775
Net issuance of common stock upon exercise of stock options, shares	52
Share-based compensation		5,212					5,212
Share repurchases, including fees	$ (11)	(16,895)		(370,360)			$ (387,266)
Share repurchases, including fees, shares	(1,025)						(1,025,000)
Balance at end of period at Sep. 30, 2019	$ 757	1,259,544		(1,094,082)	0		$ 166,219
Balance (in shares) at Sep. 30, 2019	75,728
Balance at beginning of period at Dec. 31, 2019	$ 756	1,280,760		(889,066)	4,890		$ 397,340	[1]
Balance (in shares) at Dec. 31, 2019	75,619						75,618,659
Net income				1,359,357			$ 1,359,357
Total other comprehensive income (loss)					(21,925)		(21,925)
Issuance of common stock under employee benefit plans, net of forfeitures and shares withheld to cover taxes		12,979					12,979
Issuance of common stock under employee benefit plans, net of forfeitures and shares withheld to cover taxes, shares	36
Net issuance of common stock upon exercise of stock options	$ 3	39,503					39,506
Net issuance of common stock upon exercise of stock options, shares	251
Share-based compensation		15,968					15,968
Share repurchases, including fees	$ (26)	(45,511)		(1,048,463)			$ (1,094,000)
Share repurchases, including fees, shares	(2,634)						(2,634,000)
Balance at end of period at Sep. 30, 2020	$ 733	1,303,699		(578,172)	(17,035)		$ 709,225
Balance (in shares) at Sep. 30, 2020	73,272						73,272,379
Balance at beginning of period at Jun. 30, 2020	$ 741	1,289,976		(679,506)	(22,111)		$ 589,100
Balance (in shares) at Jun. 30, 2020	74,098
Net income				527,252			527,252
Total other comprehensive income (loss)					5,076		5,076
Issuance of common stock under employee benefit plans, net of forfeitures and shares withheld to cover taxes		4,057					4,057
Issuance of common stock under employee benefit plans, net of forfeitures and shares withheld to cover taxes, shares	9
Net issuance of common stock upon exercise of stock options	$ 1	21,522					21,523
Net issuance of common stock upon exercise of stock options, shares	130
Share-based compensation		5,190					5,190
Share repurchases, including fees	$ (9)	(17,046)		(425,918)			$ (442,973)
Share repurchases, including fees, shares	(965)						(965,000)
Balance at end of period at Sep. 30, 2020	$ 733	$ 1,303,699		$ (578,172)	$ (17,035)		$ 709,225
Balance (in shares) at Sep. 30, 2020	73,272						73,272,379

[1] The balance sheet at December 31, 2019, has been derived from the audited consolidated financial statements at that date but does not include all of the information and footnotes required by United States generally accepted accounting principles for complete financial statements.