Earnings Per Share (details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Earnings per share of common stock [Abstract]
Net income	$ 29,244	$ (8,312)	$ 2,193	$ 40,773	$ 29,591	$ (7,420)	$ 6,888	$ 43,608	$ 63,898	$ 72,667	$ 75,122
Average common shares outstanding - basic									26,687	26,589	26,511
Additional shares for stock based compensation plans									57	68	65
Average common shares outstanding - diluted									26,744	26,657	26,576
Earnings per share of common stock - basic	$ 1.09	$ (0.31)	$ 0.08	$ 1.53	$ 1.11	$ (0.28)	$ 0.26	$ 1.64	$ 2.39	$ 2.73	$ 2.83
Earnings per share of common stock - diluted	$ 1.09	$ (0.31)	$ 0.08	$ 1.53	$ 1.11	$ (0.28)	$ 0.26	$ 1.64	$ 2.39	$ 2.73	$ 2.83
Antidilutive securities excluded from computation of earnings per share Amount									2,101	743	2,142