Stock-Based Compensation - Summary of Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Stock option activity
Balance at the beginning of the period (in shares)	510,574	572,629	568,727
Granted (in shares)	0	35,800	89,575
Exercised (in shares)	(38,418)	(97,855)	(79,974)
Forfeited (in shares)		0	(5,699)
Balance at the end of the period (in shares)	472,156	510,574	572,629
Vested and Expected to Vest (in shares)	468,601
Exercisable at the end of the period(in shares)	394,363
Weighted average exercise price
Balance at the beginning of the period (in dollars per share)	$ 30.60	$ 24.42	$ 16.62
Granted (in dollars per share)		85.99	69.93
Exercised (in dollars per share)	32.94	14.75	18.79
Forfeited (in dollars per share)		0.00	39.83
Balance at the end of the period (in dollars per share)	30.41	$ 30.60	$ 24.42
Vested and Expected to Vest (in dollars per share)	30.05
Exercisable at the end of the period (in dollars per share)	$ 21.86
Weighted average remaining contractual life
Outstanding at the beginning of period	3 years 259 days
Vested and Expected to Vest	3 years 248 days
Exercisable at the end of period	2 years 350 days
Aggregate intrinsic value
Outstanding at the end of the period	$ 26,934
Vested and Expected to Vest at the end of the period	26,897
Exercisable at the end of the period	$ 25,864