Employee Stock Plans - Stock Option Activity Table (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Shares outstanding	1,540,369	1,744,345
Shares outstanding, weighted-average exercise price	$ 58.34	$ 50.72
Shares granted	378,753
Shares granted, weighted-average exercise price	$ 73.89
Shares exercised	(508,533)
Shares exercised, weighted-average exercise price	$ 43.65
Shares forfeited	(74,196)
Shares forfeited, weighted-average exercise price	$ 59.24
Shares exercisable	619,083
Shares exercisable, weighted-average exercise price	$ 48.95
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Exercisable, Weighted Average Remaining Contractual Term	65 months
Shares outstanding, weighted-average remaining contractual life	85 months
Shares outstanding, aggregate intrinsic value	$ 33,990
Shares exercisable, aggregate intrinsic value	$ 19,477