Net Income per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Net income	$ 18,239	$ 31,344
Less: Net income allocated to unvested participating restricted stock	(329)	(796)
Less: Accretion of Series B Convertible Preferred Stock	0	(668)
Net income available to common stockholders	17,910	29,880
Effect of unvested participating restricted stock in two-class calculation	1	40
Net income available to common stockholders - diluted	17,911	30,588
Weighted average outstanding shares of common stock - basic	18,911	17,682
Dilutive effect of stock options	282	316
Weighted average outstanding shares of common stock - diluted	19,193	18,651
Net income per common share - basic	$ 0.95	$ 1.69
Net income per common share -diluted	$ 0.93	$ 1.64
Series B Preferred Stock
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Less: Accretion of Series B Convertible Preferred Stock	0	(668)
Accretion of Series B Convertible Preferred Stock	$ 0	$ 668
Dilutive effect of series B preferred stock	0	653
Common Stock
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Weighted average outstanding shares of common stock - diluted	19,193	18,651