Earnings Per Share (details) (USD $) In Thousands, except Per 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
Earnings Per Share [Abstract]
Net income	$ 28,397	$ (10,558)	$ 1,409	$ 40,607	$ 29,244	$ (8,312)	$ 2,193	$ 40,773	$ 59,855	$ 63,898	$ 72,667
Average common shares outstanding - basic									26,831	26,687	26,589
Additional shares for stock based compensation plans									76	57	68
Average common shares outstanding - diluted									26,907	26,744	26,657
Earnings per share of common stock - basic	$ 1.06	$ (0.39)	$ 0.05	$ 1.52	$ 1.09	$ (0.31)	$ 0.08	$ 1.53	$ 2.23	$ 2.39	$ 2.73
Earnings per share of common stock - diluted	$ 1.05	$ (0.39)	$ 0.05	$ 1.51	$ 1.09	$ (0.31)	$ 0.08	$ 1.53	$ 2.22	$ 2.39	$ 2.73
Antidilutive shares excluded from computation of earnings per share amount									1	2	1