Employee Benefits and Share-Based Compensation - Restricted Stock Activity (Details) - 2009 Plan - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Restricted Stock Units (RSUs)
Number of Shares
Outstanding at beginning of period (in shares)	538
Granted (in shares)	30
Vested (in shares)	(36)
Forfeited (in shares)	(27)
Outstanding and unvested at end of period (in shares)	505	538
Weighted-Average Grant Date Fair Value
Outstanding and unvested (in dollars per share)	$ 51.52
Granted (in dollars per share)	78.23
Vested (in dollars per share)	39.78
Forfeited (in dollars per share)	44.87
Outstanding and unvested (in dollars per share)	$ 54.32	$ 51.52
Weighted-average remaining years, outstanding and unvested	1 year 5 months 16 days	1 year 7 months 6 days
Aggregate Intrinsic Value
Outstanding and unvested at end of period	$ 40,813	$ 32,935
Unrecognized compensation cost	$ 23,400
Weighted average period of compensation cost not yet recognized (in years)	2 years 8 months 27 days
Restricted Stock Awards (RSAs)
Number of Shares
Outstanding at beginning of period (in shares)	21
Granted (in shares)	0
Vested (in shares)	0
Forfeited (in shares)	0
Outstanding and unvested at end of period (in shares)	21	21
Weighted-Average Grant Date Fair Value
Outstanding and unvested (in dollars per share)	$ 46.60
Granted (in dollars per share)	0.00
Vested (in dollars per share)	0.00
Forfeited (in dollars per share)	0.00
Outstanding and unvested (in dollars per share)	$ 46.60	$ 46.60
Aggregate Intrinsic Value
Unrecognized compensation cost	$ 100
Weighted average period of compensation cost not yet recognized (in years)	1 month 13 days