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	201,971				201,971
Net change in unrealized gains (losses) from investments	57			57
Net change in foreign currency translation adjustment	283			283
Issuance of common stock relating to employee equity compensation plans (in shares)		669
Issuance of common stock relating to employee equity compensation plans	8,585		8,585
Tax withholdings related to net share settlements of restricted stock units	(79,330)		(79,330)
Common stock repurchased and retired (shares)		(396)
Common stock repurchased and retired	(100,000)		(3,811)		(96,189)
Stock-based compensation	32,720		32,720
Other	385				385
Ending Balance (in shares) at Jun. 30, 2018		80,313
Ending Balance at Jun. 30, 2018	1,218,959	$ 8	844,599	911	373,441
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 Mar. 31, 2018		80,144
Beginning Balance at Mar. 31, 2018	1,127,460	$ 8	858,632	1,484	267,336
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	106,105				106,105
Net change in unrealized gains (losses) from investments	186			186
Net change in foreign currency translation adjustment	(759)			(759)
Issuance of common stock relating to employee equity compensation plans (in shares)		169
Issuance of common stock relating to employee equity compensation plans	565		565
Tax withholdings related to net share settlements of restricted stock units	(31,488)		(31,488)
Stock-based compensation	16,890		16,890
Ending Balance (in shares) at Jun. 30, 2018		80,313
Ending Balance at Jun. 30, 2018	1,218,959	$ 8	844,599	911	373,441
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	218,990				218,990
Net change in unrealized gains (losses) from investments	276			276
Net change in foreign currency translation adjustment	622			622
Issuance of common stock relating to employee equity compensation plans (in shares)		453
Issuance of common stock relating to employee equity compensation plans	9,614		9,614
Tax withholdings related to net share settlements of restricted stock units	(52,718)		(52,718)
Common stock repurchased and retired (shares)		(366)
Common stock repurchased and retired	(99,504)		(3,646)		(95,858)
Stock-based compensation	43,511		43,511
Ending Balance (in shares) at Jun. 30, 2019		79,865
Ending Balance at Jun. 30, 2019	1,373,682	$ 8	874,275	(1,876)	501,275
Beginning Balance (in shares) at Mar. 31, 2019		80,000
Beginning Balance at Mar. 31, 2019	1,255,704	$ 8	855,956	(2,281)	402,021
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	147,142				147,142
Net change in unrealized gains (losses) from investments	192			192
Net change in foreign currency translation adjustment	213			213
Issuance of common stock relating to employee equity compensation plans (in shares)		26
Issuance of common stock relating to employee equity compensation plans	5		5
Tax withholdings related to net share settlements of restricted stock units	$ (2,537)		(2,537)
Common stock repurchased and retired (shares)	(161)
Common stock repurchased and retired	$ (49,504)		(1,616)		(47,888)
Stock-based compensation	22,467		22,467
Ending Balance (in shares) at Jun. 30, 2019		79,865
Ending Balance at Jun. 30, 2019	$ 1,373,682	$ 8	$ 874,275	$ (1,876)	$ 501,275