CONDENSED CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY - USD ($) $ in Thousands	Common stock [Member]	Additional paid-in capital [Member]	Retained earnings (deficit) [Member]	Accumulated other comprehensive income (loss) [Member]	Total
Balance at beginning 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
Net income			832,105		832,105
Total other comprehensive income (loss)				(27,001)	(27,001)
Issuance of common stock under employee benefit plans, net of forfeitures and shares withheld to cover taxes		8,922			8,922
Issuance of common stock under employee benefit plans, net of forfeitures and shares withheld to cover taxes, shares	27,000
Net issuance of common stock upon exercise of stock options	$ 2	17,981			17,983
Net issuance of common stock upon exercise of stock options, shares	121,000
Share-based compensation		10,778			10,778
Share repurchases, including fees	$ (17)	(28,465)	(622,545)		$ (651,027)
Share repurchases, including fees, shares	(1,669,000)				(1,669,000)
Balance at end of period at Jun. 30, 2020	$ 741	1,289,976	(679,506)	(22,111)	$ 589,100
Balance (in shares) at Jun. 30, 2020	74,098,000
Balance at beginning of period at Mar. 31, 2020	$ 742	1,271,250	(1,137,392)	(25,756)	108,844
Balance (in shares) at Mar. 31, 2020	74,199,000
Net income			531,667		531,667
Total other comprehensive income (loss)				3,645	3,645
Issuance of common stock under employee benefit plans, net of forfeitures and shares withheld to cover taxes		4,818			4,818
Issuance of common stock under employee benefit plans, net of forfeitures and shares withheld to cover taxes, shares	15,000
Net issuance of common stock upon exercise of stock options	$ 1	11,846			11,847
Net issuance of common stock upon exercise of stock options, shares	69,000
Share-based compensation		5,254			5,254
Share repurchases, including fees	$ (2)	(3,192)	(73,781)		$ (76,975)
Share repurchases, including fees, shares	(185,000)				(185,000)
Balance at end of period at Jun. 30, 2020	$ 741	1,289,976	(679,506)	(22,111)	$ 589,100
Balance (in shares) at Jun. 30, 2020	74,098,000
Balance at beginning of period at Dec. 31, 2020	$ 711	1,280,841	(1,139,139)	(2,155)	$ 140,258	[1]
Balance (in shares) at Dec. 31, 2020	71,123,000				71,123,109
Net income			1,087,060		$ 1,087,060
Total other comprehensive income (loss)				(436)	(436)
Issuance of common stock under employee benefit plans, net of forfeitures and shares withheld to cover taxes		9,430			9,430
Issuance of common stock under employee benefit plans, net of forfeitures and shares withheld to cover taxes, shares	22,000
Net issuance of common stock upon exercise of stock options	$ 2	33,770			33,772
Net issuance of common stock upon exercise of stock options, shares	206,000
Share-based compensation		11,799			11,799
Share repurchases, including fees	$ (22)	(40,477)	(1,023,690)		$ (1,064,189)
Share repurchases, including fees, shares	(2,218,000)				(2,218,000)
Balance at end of period at Jun. 30, 2021	$ 691	1,295,363	(1,075,769)	(2,591)	$ 217,694
Balance (in shares) at Jun. 30, 2021	69,133,000				69,132,589
Balance at beginning of period at Mar. 31, 2021	$ 697	1,274,033	(1,275,409)	(6,298)	$ (6,977)
Balance (in shares) at Mar. 31, 2021	69,735,000
Net income			585,451		585,451
Total other comprehensive income (loss)				3,707	3,707
Issuance of common stock under employee benefit plans, net of forfeitures and shares withheld to cover taxes		5,340			5,340
Issuance of common stock under employee benefit plans, net of forfeitures and shares withheld to cover taxes, shares	11,000
Net issuance of common stock upon exercise of stock options	$ 1	23,926			23,927
Net issuance of common stock upon exercise of stock options, shares	130,000
Share-based compensation		5,887			5,887
Share repurchases, including fees	$ (7)	(13,823)	(385,811)		$ (399,641)
Share repurchases, including fees, shares	(743,000)				(743,000)
Balance at end of period at Jun. 30, 2021	$ 691	$ 1,295,363	$ (1,075,769)	$ (2,591)	$ 217,694
Balance (in shares) at Jun. 30, 2021	69,133,000				69,132,589

[1] The balance sheet at December 31, 2020, 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.