Stockholders' Equity (Summary of Activity Related to Stock Option Plans) (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Shares Available for Grant, Beginning Balances	10,739,915	13,289,953	16,845,316
Options Outstanding, Number of Shares, Beginning Balances	21,647,350	22,437,347	20,995,756
Options Outstanding, Number of Shares, Granted	(2,039,974)	(2,550,038)	(3,555,363)
Options Outstanding, Number of Shares, Exercised	(3,205,911)	(3,338,474)	(2,113,772)
Options Outstanding, Number of Shares, Expired	(2,135)	(1,561)
Shares Available for Grant, Ending Balances	8,699,941	10,739,915	13,289,953
Options Outstanding, Number of Shares, Ending Balances	20,479,278	21,647,350	22,437,347
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Options Outstanding, Weighted-Average Exercise Price, Beginning Balances (in USD per share)	$ 61.13	$ 44.83	$ 32.39
Options Outstanding, Weighted-Average Exercise Price, Granted (in USD per share)	311.66	159.56	102.03
Options Outstanding, Weighted-Average Exercise Price, Exercised (in USD per share)	38.66	26.79	17.48
Options Outstanding, Weighted-Average Exercise Price, Expired (in USD per share)	4.60	3.25
Options Outstanding, Weighted-Average Exercise Price, Ending Balances (in USD per share)	$ 89.61	$ 61.13	$ 44.83
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Options Outstanding, Number of Shares, Vested and exercisable	20,479,278
Options Outstanding, Weighted-Average Exercise Price, Vested and exercisable (in USD per share)	$ 89.61
Weighted-Average Remaining Contractual Term, Vested and exercisable (in Years)	5 years 8 months 16 days
Aggregate Intrinsic Value, Vested and exercisable	$ 3,748,339
Expiration period	10 years