Employee Benefits and Share-Based Compensation - Restricted Stock Units and Restricted Stock Awards (Details) - 2009 Plan - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Restricted Stock Awards
Number of Shares
Nonvested, beginning balance (in shares)	23
Granted (Awarded) (in shares)	21
Vested (Released) (in shares)	(23)
Nonvested, ending balance (in shares)	21	23
Weighted-Average Grant Date Fair Value
Nonvested, beginning balance (in dollars per share)	$ 41.07
Granted (Awarded) (in dollars per share)	46.60	$ 41.10	$ 31.59
Vested (Released) (in dollars per share)	41.07
Nonvested, ending balance (in dollars per share)	$ 46.60	$ 41.07
Aggregate Intrinsic Value
Fair value of restricted stock units	$ 1,000	$ 1,000	$ 1,200
Unrecognized compensation cost	$ 300
Weighted average period of compensation cost recognized (in years)	4 months 13 days
Restricted Stock Units (RSUs)
Number of Shares
Nonvested, beginning balance (in shares)	501
Granted (Awarded) (in shares)	312
Vested (Released) (in shares)	(213)
Forfeited (in shares)	(62)
Nonvested, ending balance (in shares)	538	501
Weighted-Average Grant Date Fair Value
Nonvested, beginning balance (in dollars per share)	$ 38.90
Granted (Awarded) (in dollars per share)	59.52	$ 45.97	$ 32.58
Vested (Released) (in dollars per share)	37.14
Forfeited (in dollars per share)	39.00
Nonvested, ending balance (in dollars per share)	$ 51.52	$ 38.90
Weighted-Average Remaining Years	1 year 6 months 22 days	1 year 6 months
Aggregate Intrinsic Value
Outstanding and unvested	$ 32,935	$ 24,293
Fair value of restricted stock units	7,900	$ 6,500	$ 4,800
Unrecognized compensation cost	$ 24,100
Weighted average period of compensation cost recognized (in years)	2 years 10 months 2 days