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, 2015	Mar. 31, 2014
Earnings Per Share [Abstract]
Net income	$ 28,412	$ 20,824
Less: Net income allocated to unvested participating restricted stock	(372)	(343)
Net income available to common stockholders	28,040	20,481
Effect of unvested participating restricted stock in two-class calculation	2	2
Net income available to common stockholders - diluted	$ 28,042	$ 20,483
Weighted average outstanding shares of common stock - basic (in shares)	18,870	18,794
Dilutive effect of stock options (in shares)	149	260
Weighted average outstanding shares of common stock - diluted (in shares)	19,019	19,054
Net income per common share - basic (USD per share)	$ 1.49	$ 1.09
Net income per common share -diluted (USD per share)	$ 1.47	$ 1.08