Stock-Based Compensation - Stock-Based Compensation Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Number of shares outstanding, beginning balance (in shares)	3,419,597	3,117,616
Number of shares granted (in shares)	608,598	743,250
Stock option exercises (in shares)	(644,610)	(371,853)
Number of shares canceled (in shares)	(187,721)	(69,416)
Number of shares outstanding, ending balance (in shares)	3,195,864	3,419,597
Number of shares, options, exercisable, number (in shares)	1,766,869
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Weighted average exercise price per share outstanding, beginning balance (in dollars per share)	$ 121.31	$ 104.54
Weighted average exercise price per share granted (in dollars per share)	210.56	172.23
Weighted average exercise price per share exercised (in dollars per share)	95.14	75.23
Weighted average exercise price per share cancelled (in dollars per share)	170.75	159.97
Weighted average exercise price per share outstanding, ending balance (in dollars per share)	140.68	$ 121.31
Weighted average exercise price per share exercisable (in dollars per share)	$ 103.48
Weighted average remaining contractual term of options outstanding	6 years 1 month 2 days	6 years 1 month 24 days
Aggregate intrinsic value of options outstanding	$ 368,589,147	$ 211,369,740
Weighted average remaining contractual term of options exercisable	4 years 1 month 13 days
Aggregate intrinsic value of options exercisable	$ 269,474,477