Condensed Consolidated Statements of Stockholders' Equity - USD ($) shares in Thousands, $ in Thousands		Total	Common Stock	Additional Paid-In Capital	Retained Earnings	Accumulated Other Comprehensive Income (Loss)
Beginning balance (in shares) at Dec. 31, 2017			73,706
Beginning balance at Dec. 31, 2017		$ 1,661,914	$ 7	$ 804,731	$ 859,114	$ (1,938)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative-effect adjustment to beginning balance		3,574			3,574
Net income		157,793			157,793
Other comprehensive income (loss), net of tax		(2,504)				(2,504)
Stock-based compensation		66,583		66,583
Issuance of common stock in connection with employee equity incentive plans (in shares)			1,634
Issuance of common stock in connection with employee equity incentive plans		49,642	$ 1	49,641
Tax withholding paid for net share settlement of equity awards (in shares)			(27)
Tax withholding paid for net share settlement of equity awards		(6,914)		(6,914)
Vesting of early-exercised stock options		233		233
Common stock issued for business acquisition (in shares)			80
Common stock issued for business acquisition		15,555		15,555
Ending balance (in shares) at Sep. 30, 2018			75,393
Ending balance at Sep. 30, 2018		1,945,876	$ 8	929,829	1,020,481	(4,442)
Beginning balance (in shares) at Jun. 30, 2018			74,791
Beginning balance at Jun. 30, 2018		1,719,973	$ 8	872,559	851,957	(4,551)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative-effect adjustment to beginning balance		0			0
Net income		168,524			168,524
Other comprehensive income (loss), net of tax		109				109
Stock-based compensation		23,254		23,254
Issuance of common stock in connection with employee equity incentive plans (in shares)			531
Issuance of common stock in connection with employee equity incentive plans		20,832	$ 0	20,832
Tax withholding paid for net share settlement of equity awards (in shares)			(9)
Tax withholding paid for net share settlement of equity awards		(2,451)		(2,451)
Vesting of early-exercised stock options		80		80
Common stock issued for business acquisition (in shares)			80
Common stock issued for business acquisition		15,555		15,555
Ending balance (in shares) at Sep. 30, 2018			75,393
Ending balance at Sep. 30, 2018		1,945,876	$ 8	929,829	1,020,481	(4,442)
Beginning balance (in shares) at Dec. 31, 2018			75,668
Beginning balance at Dec. 31, 2018		2,143,389	$ 8	956,572	1,190,803	(3,994)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative-effect adjustment to beginning balance	[1]	3,702			3,702
Net income		599,175			599,175
Other comprehensive income (loss), net of tax		4,195				4,195
Stock-based compensation		74,845		74,845
Issuance of common stock in connection with employee equity incentive plans (in shares)			1,648
Issuance of common stock in connection with employee equity incentive plans		52,177		52,177
Tax withholding paid for net share settlement of equity awards (in shares)			(29)
Tax withholding paid for net share settlement of equity awards		(7,069)		(7,069)
Vesting of early-exercised stock options		207		207
Repurchase of common stock (in shares)			(919)
Repurchase of common stock		(214,617)	$ (214,617)		(214,617)
Ending balance (in shares) at Sep. 30, 2019			76,368
Ending balance at Sep. 30, 2019		2,656,004	$ 8	1,076,732	1,579,063	201
Beginning balance (in shares) at Jun. 30, 2019			76,555
Beginning balance at Jun. 30, 2019		2,525,560	$ 8	1,038,740	1,484,777	2,035
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative-effect adjustment to beginning balance	[1]	0			0
Net income		208,895			208,895
Other comprehensive income (loss), net of tax		(1,834)				(1,834)
Stock-based compensation		26,257		26,257
Issuance of common stock in connection with employee equity incentive plans (in shares)			336
Issuance of common stock in connection with employee equity incentive plans		14,073		14,073
Tax withholding paid for net share settlement of equity awards (in shares)			(11)
Tax withholding paid for net share settlement of equity awards		(2,407)		(2,407)
Vesting of early-exercised stock options		69		69
Repurchase of common stock (in shares)			(512)
Repurchase of common stock		(114,609)	$ (114,609)		(114,609)
Ending balance (in shares) at Sep. 30, 2019			76,368
Ending balance at Sep. 30, 2019		$ 2,656,004	$ 8	$ 1,076,732	$ 1,579,063	$ 201

[1] On January 1, 2019, we adopted Accounting Standard Codification Topic 842 - Leases ("ASC 842"), which resulted in a cumulative-effect adjustment to the beginning balance of Retained Earnings for 2019. See Note 1 of the accompanying notes for further details.