Net Income per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Net income	$ 18,730	$ 60,573	$ 53,906	$ 108,855
Less: Net income allocated to unvested participating restricted stock	(296)	(1,187)	(925)	(2,477)
Less: Accretion of Series B Convertible Preferred Stock	0	(688)	0	(2,033)
Net income available to common stockholders	18,434	58,698	52,981	104,345
Effect of unvested participating restricted stock in two-class calculation	1	57	3	121
Net income available to common stockholders - diluted	18,435	59,443	52,984	106,499
Weighted average outstanding shares of common stock - basic (in shares)	18,831	18,006	18,898	17,859
Dilutive effect of stock options (in shares)	254	246	268	270
Weighted average outstanding shares of common stock - diluted (in shares)	19,085	18,924	19,166	18,801
Net income per common share - basic (per share)	$ 0.98	$ 3.26	$ 2.80	$ 5.84
Net income per common share -diluted (per share)	$ 0.97	$ 3.14	$ 2.76	$ 5.66
Series B Preferred Stock
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Accretion of Series B Convertible Preferred Stock	$ 0	$ 688	$ 0	$ 2,033
Dilutive effect of series B preferred stock (in shares)	0	672	0	672
Common Stock
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Weighted average outstanding shares of common stock - diluted (in shares)	19,085	18,924	19,166	18,801