Net Income per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Net income	$ 16,937	$ 16,938	$ 35,176	$ 48,282
Less: Net income allocated to unvested participating restricted stock	(298)	(388)	(627)	(1,169)
Less: Accretion of Series B Convertible Preferred Stock	0	(677)	0	(1,345)
Net income available to common stockholders	16,639	15,873	34,549	45,768
Effect of unvested participating restricted stock in two-class calculation	1	5	2	58
Net income available to common stockholders - diluted	16,640	15,878	34,551	47,171
Weighted average outstanding shares of common stock - basic	18,953,000	17,890,000	18,932,000	17,786,000
Dilutive effect of stock options	269,000	248,000	275,000	282,000
Weighted average outstanding shares of common stock - diluted	19,222,000	18,138,000	19,207,000	18,731,000
Net income per common share - basic	$ 0.88	$ 0.89	$ 1.82	$ 2.57
Net income per common share -diluted	$ 0.87	$ 0.88	$ 1.80	$ 2.52
Common stock equivalents excluded from diluted income per common share		662,500
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	(677)	0	(1,345)
Accretion of Series B Convertible Preferred Stock	$ 0	$ 0	$ 0	$ 1,345
Dilutive effect of series B preferred stock	0	0	0	663,000
Common Stock
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Weighted average outstanding shares of common stock - diluted	19,222,000	18,138,000	19,207,000	18,731,000