Equity Incentive Plans - Summary of option activity (Details) - Stock options - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Stock option activity
Outstanding at the beginning of the period (in shares)	13,560
Granted (in shares)	2,631
Exercised (in shares)	(4,688)
Forfeited (in shares)	(229)
Expired (in shares)	(12)
Outstanding at the end of the period (in shares)	11,262	13,560
Outstanding, net of expected forfeitures (in shares)	11,140
Outstanding, exercisable ("vested") (in shares)	6,314
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Outstanding at the beginning of the period (in dollars per share)	$ 50.01
Granted (in dollars per share)	78.93
Exercised (in dollars per share)	44.91
Forfeited (in dollars per share)	70.85
Expired (in dollars per share)	59.91
Outstanding at the end of the period (in dollars per share)	58.46	$ 50.01
Outstanding, net of expected forfeitures (in dollars per share)	58.27
Outstanding, exercisable ("vested") (in dollars per share)	$ 47.83
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Aggregate intrinsic value, Outstanding	$ 520,900
Aggregate intrinsic value, net of expected forfeitures, outstanding	517,276
Aggregate intrinsic value, exercisable ("vested"), outstanding	$ 359,121
Weighted average remaining contractual term, Outstanding	6 years 6 months
Weighted average remaining contractual term, net of expected forfeitures, outstanding	6 years 6 months
Weighted average remaining contractual term, exercisable ("vested"), outstanding	5 years 1 month 6 days
Weighted average grant date fair value of options granted (in dollars per share)	$ 14.60	$ 12.25	$ 15.45
Intrinsic value of options exercised	$ 199,000	$ 119,000	$ 117,000