Common Stock (Basic and Diluted Earnings Per Share) (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Common Stock [Abstract]
Basic number of common shares outstanding									39,974,022	41,349,883	43,129,735
Dilutive effect of unvested performance awards									66,756	206,658	101,780
Dilutive effect of stock options									14,803	30,518	62,904
Diluted number of common shares outstanding									40,055,581	41,587,059	43,294,419
Net income	$ 4,819	$ 51,789	$ 30,894	$ 3,344	$ 5,453	$ 58,321	$ 32,990	$ 6,775	$ 90,846	$ 103,539	$ 100,603
Income allocated to participating restricted stock									(256)	(471)	(403)
Net income available to common shareholders basic									90,590	103,068	100,200
Income reallocated to participating restricted stock									(256)	(469)	(401)
Net income available to common shareholders diluted									$ 90,590	$ 103,070	$ 100,202
Earnings Per Share, Basic, Distributed									$ 0.97	$ 0.66	$ 0.00
Earnings Per Share, Basic, Undistributed									$ 1.30	$ 1.83	$ 2.32
Basic net income per common share	$ 0.12	$ 1.29	$ 0.77	$ 0.08	$ 0.14	$ 1.41	$ 0.78	$ 0.16	$ 2.27	$ 2.49	$ 2.32
Earnings Per Share, Diluted, Distributed									$ 0.97	$ 0.66	$ 0.00
Earnings Per Share, Diluted, Undistributed									$ 1.29	$ 1.82	$ 2.31
Diluted net income per common share	$ 0.12	$ 1.29	$ 0.77	$ 0.08	$ 0.13	$ 1.40	$ 0.78	$ 0.16	$ 2.26	$ 2.48	$ 2.31