CONDENSED CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Income (Loss), Net	Retained Earnings
Beginning Balance (in shares) at Dec. 31, 2017		80,040
Beginning Balance at Dec. 31, 2017	$ 1,154,288	$ 8	$ 886,435	$ 571	$ 267,274
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	302,843				302,843
Net change in unrealized gains (losses) from investments	174			174
Net change in foreign currency translation adjustment	(1,473)			(1,473)
Issuance of common stock relating to employee equity compensation plans (in shares)		765
Issuance of common stock relating to employee equity compensation plans	16,607		16,607
Tax withholdings related to net share settlements of restricted stock units	(81,756)		(81,756)
Common stock repurchased and retired (shares)		(822)
Common stock repurchased and retired	(250,002)		(7,933)		(242,069)
Stock-based compensation	51,918		51,918
Other	384				384
Ending Balance (in shares) at Sep. 30, 2018		79,983
Ending Balance at Sep. 30, 2018	1,192,983	$ 8	865,271	(728)	328,432
Beginning Balance (in shares) at Dec. 31, 2017		80,040
Beginning Balance at Dec. 31, 2017	1,154,288	$ 8	886,435	571	267,274
Ending Balance (in shares) at Dec. 31, 2018		79,778
Ending Balance at Dec. 31, 2018	1,252,891	$ 8	877,514	(2,774)	378,143
Beginning Balance (in shares) at Jun. 30, 2018		80,313
Beginning Balance at Jun. 30, 2018	1,218,959	$ 8	844,599	911	373,441
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	100,872				100,872
Net change in unrealized gains (losses) from investments	117			117
Net change in foreign currency translation adjustment	(1,756)			(1,756)
Issuance of common stock relating to employee equity compensation plans (in shares)		96
Issuance of common stock relating to employee equity compensation plans	8,021		8,021
Tax withholdings related to net share settlements of restricted stock units	(2,426)		(2,426)
Common stock repurchased and retired (shares)		(426)
Common stock repurchased and retired	(150,002)		(4,121)		(145,881)
Stock-based compensation	19,198		19,198
Ending Balance (in shares) at Sep. 30, 2018		79,983
Ending Balance at Sep. 30, 2018	1,192,983	$ 8	865,271	(728)	328,432
Beginning Balance (in shares) at Dec. 31, 2018		79,778
Beginning Balance at Dec. 31, 2018	1,252,891	$ 8	877,514	(2,774)	378,143
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	321,514				321,514
Net change in unrealized gains (losses) from investments	317			317
Net change in foreign currency translation adjustment	530			530
Issuance of common stock relating to employee equity compensation plans (in shares)		529
Issuance of common stock relating to employee equity compensation plans	17,907		17,907
Tax withholdings related to net share settlements of restricted stock units	(55,793)		(55,793)
Common stock repurchased and retired (shares)		(1,498)
Common stock repurchased and retired	(299,504)		(15,006)		(284,498)
Stock-based compensation	67,687		67,687
Ending Balance (in shares) at Sep. 30, 2019		78,809
Ending Balance at Sep. 30, 2019	1,305,549	$ 8	892,309	(1,927)	415,159
Beginning Balance (in shares) at Jun. 30, 2019		79,865
Beginning Balance at Jun. 30, 2019	1,373,682	$ 8	874,275	(1,876)	501,275
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	102,524				102,524
Net change in unrealized gains (losses) from investments	41			41
Net change in foreign currency translation adjustment	(92)			(92)
Issuance of common stock relating to employee equity compensation plans (in shares)		76
Issuance of common stock relating to employee equity compensation plans	8,293		8,293
Tax withholdings related to net share settlements of restricted stock units	$ (3,075)		(3,075)
Common stock repurchased and retired (shares)	(1,132)
Common stock repurchased and retired	$ (200,000)		(11,360)		(188,640)
Stock-based compensation	24,176		24,176
Ending Balance (in shares) at Sep. 30, 2019		78,809
Ending Balance at Sep. 30, 2019	$ 1,305,549	$ 8	$ 892,309	$ (1,927)	$ 415,159