Selected Quarterly Financial Data (Unaudited) (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
Quarterly Financial Information Disclosure [Abstract]
Operating Revenues	$ 240,600		$ 238,763		$ 242,709		$ 197,486		$ 180,001		$ 166,040		$ 164,457		$ 166,012		$ 919,558		$ 676,510
Net (Loss) Earnings	$ (414,733)		$ 134,001		$ 105,166		$ (52,497)		$ (21,145)		$ (3,023)		$ 89,023		$ 17,669		$ (228,063)		$ 82,524		$ 54,030
Basic Net (Loss) Earnings Per Share (in dollars per share)	$ (7.62)	[1]	$ 2.45	[1]	$ 1.93	[1]	$ (0.98)	[1]	$ (0.40)	[1]	$ (0.06)	[1]	$ 1.65	[1]	$ 0.34	[1]	$ (4.21)	[1]	$ 1.54	[1]	$ 1.18
Dilutive Net (Loss) Earnings Per Share (in dollars per share)	$ (7.62)	[1]	$ 2.42	[1]	$ 1.90	[1]	$ (0.98)	[1]	$ (0.40)	[1]	$ (0.06)	[1]	$ 1.64	[1]	$ 0.34	[1]	$ (4.21)	[1]	$ 1.52	[1]	$ 1.17

[1] The sum of the individual quarterly net (loss) earnings per common share amounts may not agree with year-to-date net (loss) earnings per common share as each quarterly computation is based on the weighted-average number of common shares outstanding during that period. Potentially dilutive securities were included in the computation of diluted net (loss) earnings per common share for each quarter in which the Company reported net