Supplemental Financial Information (Unaudited) (Details) (USD $) Share data 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
Operating revenues	$ 1,081,144,000		$ 1,173,518,000		$ 967,962,000		$ 852,807,000	$ 1,065,749,000		$ 1,152,181,000	$ 930,757,000	$ 901,805,000	$ 4,075,431,000		$ 4,050,492,000		$ 3,909,695,000
Operating expenses	1,190,673,000	[1]	1,207,553,000	[2]	876,248,000	[3]	781,750,000	939,172,000		1,032,760,000	848,454,000	823,739,000	4,056,224,000		3,644,125,000		3,498,968,000
Operating income	(109,529,000)	[1]	(34,035,000)	[2]	91,714,000	[3]	71,057,000	126,577,000		119,421,000	82,303,000	78,066,000	19,207,000		406,367,000		410,727,000
Income (loss) from continuing operations	(69,686,000)	[1]	(29,532,000)	[2]	49,007,000	[3]	35,890,000	74,088,000		64,100,000	45,235,000	42,529,000	(14,321,000)		225,952,000		244,020,000
Income (loss) from discontinued operations, net of tax	8,700,000	[1]	(139,000)		5,106,000	[3]	(100,000)	(13,080,000)	[1]	(126,000)	(168,000)	448,000	13,567,000	[4]	(12,926,000)	[4]	(3,361,000)	[4]
Net income (loss)	(60,986,000)	[1]	(29,671,000)	[2]	54,113,000	[3]	35,790,000	61,008,000	[1]	63,974,000	45,067,000	42,977,000	(754,000)		213,026,000		240,659,000
Earnings (loss) per common share - basic [Abstract]
Earnings (loss) before discontinued operations	$ (0.37)	[1]	$ (0.16)	[2]	$ 0.26	[3]	$ 0.19	$ 0.39		$ 0.34	$ 0.24	$ 0.22	$ (0.08)		$ 1.19		$ 1.29
Discontinued operations, net of tax	$ 0.05	[1]	$ 0.00		$ 0.03	[3]	$ 0.00	$ (0.07)	[1]	$ 0.00	$ 0.00	$ 0.01	$ 0.07		$ (0.07)		$ (0.01)
Earnings (loss) per common share - basic	$ (0.32)	[1]	$ (0.16)	[2]	$ 0.29	[3]	$ 0.19	$ 0.32	[1]	$ 0.34	$ 0.24	$ 0.23	$ (0.01)		$ 1.12		$ 1.28
Earnings per common share - diluted [Abstract]
Earnings (loss) before discontinued operations	$ (0.37)	[1]	$ (0.16)	[2]	$ 0.26	[3]	$ 0.19	$ 0.39		$ 0.34	$ 0.24	$ 0.22	$ (0.08)		$ 1.19		$ 1.29
Discontinued operations, net of tax	$ 0.05	[1]	$ 0.00		$ 0.03	[3]	$ 0.00	$ (0.07)	[1]	$ 0.00	$ 0.00	$ 0.01	$ 0.07		$ (0.07)		$ (0.02)
Earnings (loss) per common share - diluted	$ (0.32)	[1]	$ (0.16)	[2]	$ 0.29	[3]	$ 0.19	$ 0.32	[1]	$ 0.34	$ 0.24	$ 0.23	$ (0.01)		$ 1.12		$ 1.27
Weighted average common shares outstanding - basic	188,831		188,831		188,831		188,811	188,794		188,794	188,794	188,671	188,826		188,763		188,137
Weighted average common shares outstanding - diluted	188,831		188,831		189,107		189,182	188,932		188,797	188,968	188,815	188,826		188,905		188,229
Reversal of an arbitration charge after tax					15,000,000								15,000,000
Noncash write-downs of oil and natural gas properties, after tax	145,900,000		100,900,000										246,800,000
Arbitration charge after tax	$ (13,000,000)							$ 13,000,000							$ 13,000,000

[1] 2012 reflects a $145.9 million after-tax noncash write-down of oil and natural gas properties and the reversal of an arbitration charge of $13.0 million (after tax) related to a guarantee of a construction contract, which was partially offset by the reversal of estimated insurance recoveries, as previously discussed. 2011 reflects an arbitration charge of $13.0 million (after tax) related to a guarantee of a construction contract. For more information, see Notes 1 and 19, respectively.

[2] 2012 reflects a $100.9 million after-tax noncash write-down of oil and natural gas properties. For more information, see Note 1.

[3] 2012 reflects a net benefit of $15.0 million (after tax) related to natural gas gathering operations litigation and a net benefit largely related to estimated insurance recoveries related to the guarantee of a construction contract. For more information, see Note 19.

[4] Reflected in the Other category.