Net Income Per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Earnings Per Share [Abstract]
Net income	$ (22,425)	$ 18,887	$ 19,167	$ 20,824	$ 18,131	$ 18,730	$ 16,937	$ 18,239	$ 36,453	$ 72,037	$ 127,674
Less: Accretion of Series B preferred stock									0	0	(2,498)
Less: Net income allocated to unvested participating restricted stock									(521)	(1,200)	(2,718)
Net income available to common stockholders - basic									35,932	70,837	122,458
Effect of unvested participating restricted stock									0	4	127
Effect of dilutive securities, convertible Series B preferred stock									0	0	2,498
Numerator - net income available to common shareholders - diluted									$ 35,932	$ 70,841	$ 125,083
Weighted average outstanding shares of common stock - basic									18,753	18,871	17,992
Stock options									203	270	264
Convertible Series B preferred stock									0	0	621
Weighted average outstanding shares of common stock - diluted									18,956	19,141	18,877
Basic (USD per share)	$ (1.18)	$ 0.99	$ 1.00	$ 1.09	$ 0.95	$ 0.98	$ 0.88	$ 0.95	$ 1.92	$ 3.75	$ 6.81
Diluted (USD per share)		$ 0.99	$ 1.00	$ 1.08	$ 0.94	$ 0.97	$ 0.87	$ 0.93	$ 1.90	$ 3.70	$ 6.63