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
	Mar. 31, 2014	Mar. 31, 2013
Earnings Per Share [Abstract]
Net income	$ 20,824	$ 18,239
Less: Net income allocated to unvested participating restricted stock	(343)	(329)
Net income available to common stockholders	20,481	17,910
Effect of unvested participating restricted stock in two-class calculation	2	1
Net income available to common stockholders - diluted	$ 20,483	$ 17,911
Weighted average outstanding shares of common stock - basic (in shares)	18,794	18,911
Dilutive effect of stock options (in shares)	260	282
Weighted average outstanding shares of common stock - diluted (in shares)	19,054	19,193
Net income per common share - basic (per share)	$ 1.09	$ 0.95
Net income per common share -diluted (per share)	$ 1.08	$ 0.93