Net Income per Share - Computation of Basic and Diluted Earnings Per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Earnings Per Share [Abstract]
Net income	$ 19,167	$ 16,937	$ 39,991	$ 35,176
Less: Net income allocated to unvested participating restricted stock	(307)	(298)	(649)	(627)
Net income available to common stockholders	18,860	16,639	39,342	34,549
Effect of unvested participating restricted stock in two-class calculation	1	1	2	2
Net income available to common stockholders - diluted	$ 18,861	$ 16,640	$ 39,344	$ 34,551
Weighted average outstanding shares of common stock - basic (in shares)	18,776	18,953	18,785	18,932
Dilutive effect of stock options (in shares)	179	269	218	275
Weighted average outstanding shares of common stock - diluted (in shares)	18,955	19,222	19,003	19,207
Net income per common share - basic (per share)	$ 1.00	$ 0.88	$ 2.09	$ 1.82
Net income per common share -diluted (per share)	$ 1.00	$ 0.87	$ 2.07	$ 1.80