Net Income per Share - Computation of Basic and Diluted Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Earnings Per Share [Abstract]
Net income	$ 17,073	$ 15,595
Less: Net income allocated to unvested participating restricted stock	(568)	(283)
Net income available to common stockholders - basic	16,505	15,312
Effect of unvested participating restricted stock in two-class calculation	2	0
Net income available to common stockholders - diluted	$ 16,507	$ 15,312
Weighted average outstanding shares of common stock - basic (in shares)	17,703	17,694
Dilutive effect of stock options (in shares)	142	43
Weighted average outstanding shares of common stock - diluted (in shares)	17,845	17,737
Net income per common share - basic (USD per share)	$ 0.93	$ 0.87
Net income per common share - diluted (USD per share)	$ 0.92	$ 0.86