Stock Compensation - Employee Stock Purchase Plan (Details) - Employee stock purchase plan	12 Months Ended
	Dec. 31, 2017 $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Options outstanding, Beginning balance (in shares) | shares	283,400
Granted (in shares) | shares	984,536
Exercised (in shares) | shares	(1,065,757)
Options outstanding, Ending balance (in shares) | shares	202,179
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Weighted-average exercise price, Outstanding, Beginning of period (in dollars per share) | $ / shares	$ 62.55
Weighted-average exercise price, Granted (in dollars per share) | $ / shares	55.19
Weighted-average exercise price, Exercised (in dollars per share) | $ / shares	67.62
Weighted-average exercise price, Outstanding, End of period (in dollars per share) | $ / shares	$ 89.74