Quarterly Financial Information (Unaudited) (Details) (USD $) Share data in Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Operating revenues	$ 1,184,423,000	$ 1,285,782,000	$ 1,060,595,000	$ 931,604,000	$ 1,081,144,000	$ 1,173,518,000	$ 967,962,000	$ 852,807,000	$ 4,462,404,000		$ 4,075,431,000		$ 4,050,492,000
Total operating expenses	1,037,306,000	1,135,909,000	969,217,000	827,073,000	1,190,673,000	1,207,553,000	876,248,000	781,750,000	3,969,505,000		4,056,224,000		3,644,125,000
Operating income	147,117,000	149,873,000	91,378,000	104,531,000	(109,529,000)	(34,035,000)	91,714,000	71,057,000	492,899,000		19,207,000		406,367,000
Income from continuing operations	91,348,000	84,550,000	46,392,000	56,592,000	(69,686,000)	(29,532,000)	49,007,000	35,890,000	278,882,000		(14,321,000)		225,952,000
Income (loss) from discontinued operations, net of tax	(58,000)	(118,000)	(59,000)	(77,000)	8,700,000	(139,000)	5,106,000	(100,000)	(312,000)	[1]	13,567,000	[1]	(12,926,000)	[1]
Net income (loss) attributable to parent	91,450,000	84,456,000	46,512,000	56,515,000	(60,986,000)	(29,671,000)	54,113,000	35,790,000
Earnings (loss) per common share - basic [Abstract]
Earnings before discontinued operations	$ 0.48	$ 0.45	$ 0.25	$ 0.30	$ (0.37)	$ (0.16)	$ 0.26	$ 0.19	$ 1.47		$ (0.08)		$ 1.19
Discontinued operations, net of tax	$ 0.00	$ 0.00	$ 0.00	$ 0.00	$ 0.05	$ 0.00	$ 0.03	$ 0.00	$ 0.00		$ 0.07		$ (0.07)
Earnings (loss) per common share - basic	$ 0.48	$ 0.45	$ 0.25	$ 0.30	$ (0.32)	$ (0.16)	$ 0.29	$ 0.19	$ 1.47		$ (0.01)		$ 1.12
Earnings per common share - diluted [Abstract]
Earnings before discontinued operations	$ 0.48	$ 0.44	$ 0.24	$ 0.30	$ (0.37)	$ (0.16)	$ 0.26	$ 0.19	$ 1.47		$ (0.08)		$ 1.19
Discontinued operations, net of tax	$ 0.00	$ 0.00	$ 0.00	$ 0.00	$ 0.05	$ 0.00	$ 0.03	$ 0.00	$ 0.00		$ 0.07		$ (0.07)
Earnings (loss) per common share - diluted	$ 0.48	$ 0.44	$ 0.24	$ 0.30	$ (0.32)	$ (0.16)	$ 0.29	$ 0.19	$ 1.47		$ (0.01)		$ 1.12
Weighted average common shares outstanding - basic	188,929	188,831	188,831	188,831	188,831	188,831	188,831	188,811	188,855		188,826		188,763
Weighted average common shares outstanding - diluted	189,766	189,638	189,463	189,222	188,831	188,831	189,107	189,182	189,693		188,826		188,905
Unrealized gain (loss) on commodity derivatives	(500,000)	(7,900,000)	8,200,000	(3,700,000)	(200,000)	(700,000)	3,000,000	(2,600,000)
Asset impairment charges			9,000,000				1,700,000		9,000,000		1,700,000
Reversal of an arbitration charge after tax	1,500,000				13,000,000		15,000,000		1,500,000		15,000,000
Noncash write-downs of oil and natural gas properties, after tax					$ 145,900,000	$ 100,900,000					$ 246,800,000

[1] Reflected in the Other category.