Stock-based Compensation - Stock Option (Details) - Stock options - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Stock-based compensation expense
Shares available for future issuance	958,345
Shares
Outstanding at the beginning of the year (in shares)	1,132,618
Options granted (in shares)	242,500
Options exercised (in shares)	(8,041)
Outstanding at the end of the year (in shares)	1,367,077
Exercisable at the end of the year (in shares)	673,303
Weighted average exercise price
Outstanding at the beginning of the year (in dollars per share)	$ 48.52
Options granted (in dollars per share)	126.45
Options exercised (in dollars per share)	10.36
Outstanding at the end of the year (in dollars per share)	62.57
Exercisable at the end of the year (in dollars per share)	$ 20.31
Additional disclosures
Proceeds resulting from exercise of stock options	$ 0.1	$ 0.3
Weighted-average grant date fair value of options (in dollars per share)	$ 101.08	$ 96.57
Maximum
Stock-based compensation expense
Expiration period	10 years