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, 2016	Mar. 31, 2015
Earnings Per Share [Abstract]
Net income	$ 25,543	$ 28,412
Less: Net income allocated to unvested participating restricted stock	(382)	(372)
Net income available to common stockholders	25,161	28,040
Effect of unvested participating restricted stock in two-class calculation	1	2
Net income available to common stockholders - diluted	$ 25,162	$ 28,042
Weighted average outstanding shares of common stock - basic (in shares)	18,260	18,870
Dilutive effect of stock options (in shares)	113	149
Weighted average outstanding shares of common stock - diluted (in shares)	18,373	19,019
Net income per common share - basic (USD per share)	$ 1.38	$ 1.49
Net income per common share -diluted (USD per share)	$ 1.37	$ 1.47