Business Segments (Segment Information) (Details) - USD ($)	3 Months Ended									12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014		Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014		Dec. 31, 2013
Revenues From External Customers And Long Lived Assets [Line Items]
Segment income (loss)	$ (587,100,000)	$ (1,595,400,000)	$ (73,800,000)	$ (14,500,000)	$ 375,200,000		$ 245,700,000	$ 129,400,000	$ 155,300,000	$ (2,270,833,000)	$ 905,611,000		$ 1,123,473,000
Revenues from external customers										3,033,080,000	5,476,084,000		5,390,089,000
Income tax expense (benefit)										(1,026,490,000)	227,297,000		584,550,000
Depreciation, depletion and amortization										1,619,824,000	1,906,247,000		1,553,394,000
Accretion of asset retirement obligations										48,665,000	50,778,000		48,996,000
Amortization of undeveloped leases										75,312,000	74,438,000		66,891,000
Impairment of assets										2,493,156,000	51,314,000		21,587,000
Total assets at year end	11,493,812,000				16,723,738,000	[1]				11,493,812,000	16,723,738,000	[1]
Discontinued Operations [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Segment income (loss)										(15,000,000)	(119,400,000)		235,400,000
Revenues from external customers										0	0		0
Interest income										0	0		0
Interest expense										0	0		0
Income tax expense (benefit)										0	0		0
Depreciation, depletion and amortization										0	0		0
Accretion of asset retirement obligations										0	0		0
Amortization of undeveloped leases										0	0		0
Impairment of assets										0	0		0
Deferred and noncurrent income taxes										0	0		0
Additions to property, plant, equipment										0	0		8,100,000
Total assets at year end	38,300,000				427,100,000					38,300,000	427,100,000		1,325,100,000
United States [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Revenues from external customers										1,260,000,000	2,201,500,000		1,798,500,000
Impairment of assets										328,982,000	14,267,000
Canada [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Revenues from external customers										557,300,000	1,052,400,000		1,150,200,000
Impairment of assets					37,047,000
Malaysia [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Revenues from external customers										1,210,900,000	2,233,000,000		2,337,500,000
Impairment of assets										1,480,600,000
Other [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Revenues from external customers										4,900,000	(10,800,000)		103,900,000
Corporate and Other [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Segment income (loss)										(244,900,000)	(164,800,000)		(140,700,000)
Revenues from external customers										98,400,000	53,400,000		77,500,000
Interest income										4,000,000	7,700,000		3,900,000
Interest expense										117,400,000	115,800,000		71,900,000
Income tax expense (benefit)										84,500,000	(58,400,000)		(71,700,000)
Depreciation, depletion and amortization										11,900,000	8,700,000		9,800,000
Accretion of asset retirement obligations										0	0		0
Amortization of undeveloped leases										0	0		0
Impairment of assets										0	0		0
Deferred and noncurrent income taxes										(60,500,000)	(18,800,000)		(15,700,000)
Additions to property, plant, equipment										59,900,000	14,500,000		15,500,000
Total assets at year end	592,200,000				1,773,900,000					592,200,000	1,773,900,000		1,265,200,000
Operating Segments [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Segment income (loss)										(2,270,800,000)	905,600,000		1,123,500,000
Revenues from external customers										3,033,100,000	5,476,100,000		5,390,100,000
Interest income										4,000,000	7,700,000		3,900,000
Interest expense										117,400,000	115,800,000		71,900,000
Income tax expense (benefit)										(1,026,500,000)	227,300,000		584,600,000
Depreciation, depletion and amortization										1,619,800,000	1,906,200,000		1,553,400,000
Accretion of asset retirement obligations										48,700,000	50,800,000		49,000,000
Amortization of undeveloped leases										75,400,000	74,400,000		66,900,000
Impairment of assets										2,493,200,000	51,300,000		21,600,000
Deferred and noncurrent income taxes										(978,000,000)	(170,900,000)		158,100,000
Additions to property, plant, equipment										1,791,500,000	3,317,800,000		3,532,200,000
Total assets at year end	11,493,800,000				16,742,300,000					11,493,800,000	16,742,300,000		17,509,500,000
Exploration and production [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Segment income (loss)										(2,010,900,000)	1,189,800,000		1,028,800,000
Revenues from external customers										2,934,700,000	5,422,700,000		5,312,600,000
Interest income										0	0		0
Interest expense										0	0		0
Income tax expense (benefit)										(1,111,000,000)	285,700,000		656,300,000
Depreciation, depletion and amortization										1,607,900,000	1,897,500,000		1,543,600,000
Accretion of asset retirement obligations										48,700,000	50,800,000		49,000,000
Amortization of undeveloped leases										75,400,000	74,400,000		66,900,000
Impairment of assets										2,493,200,000	51,300,000		21,600,000
Deferred and noncurrent income taxes										(917,500,000)	(152,100,000)		173,800,000
Additions to property, plant, equipment										1,731,600,000	3,303,300,000		3,508,600,000
Total assets at year end	10,863,300,000				14,541,300,000					10,863,300,000	14,541,300,000		14,919,200,000
Exploration and production [Member] | United States [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Segment income (loss)										(615,700,000)	387,100,000		435,400,000
Revenues from external customers										1,253,600,000	2,196,400,000		1,803,800,000
Interest income										0	0		0
Interest expense										0	0		0
Income tax expense (benefit)										(337,000,000)	214,800,000		241,600,000
Depreciation, depletion and amortization										794,900,000	840,700,000		576,300,000
Accretion of asset retirement obligations										20,200,000	17,500,000		13,500,000
Amortization of undeveloped leases										59,200,000	50,100,000		30,300,000
Impairment of assets										329,000,000	14,300,000		0
Deferred and noncurrent income taxes										(187,700,000)	39,700,000		99,600,000
Additions to property, plant, equipment										1,263,100,000	2,028,700,000		1,785,900,000
Total assets at year end	5,717,800,000				5,745,700,000					5,717,800,000	5,745,700,000		4,530,000,000
Exploration and production [Member] | Canada [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Segment income (loss)										(583,400,000)	156,500,000		180,800,000
Revenues from external customers										549,700,000	1,044,100,000		1,144,700,000
Interest income										0	0		0
Interest expense										0	0		0
Income tax expense (benefit)										(188,800,000)	64,200,000		57,800,000
Depreciation, depletion and amortization										261,900,000	316,700,000		374,600,000
Accretion of asset retirement obligations										12,600,000	15,200,000		16,200,000
Amortization of undeveloped leases										14,400,000	19,400,000		21,000,000
Impairment of assets										683,600,000	37,000,000		21,600,000
Deferred and noncurrent income taxes										(146,000,000)	43,300,000		26,100,000
Additions to property, plant, equipment										184,900,000	445,900,000		334,500,000
Total assets at year end	2,460,600,000				3,769,800,000					2,460,600,000	3,769,800,000		4,087,800,000
Exploration and production [Member] | Malaysia [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Segment income (loss)										(653,200,000)	896,200,000		786,400,000
Revenues from external customers										1,131,400,000	2,183,500,000		2,280,500,000
Interest income										0	0		0
Interest expense										0	0		0
Income tax expense (benefit)										(567,900,000)	102,600,000		477,700,000
Depreciation, depletion and amortization										544,900,000	735,000,000		588,200,000
Accretion of asset retirement obligations										15,900,000	18,100,000		15,000,000
Amortization of undeveloped leases										0	0		0
Impairment of assets										1,480,600,000	0		0
Deferred and noncurrent income taxes										(579,200,000)	(235,100,000)		48,100,000
Additions to property, plant, equipment										244,400,000	818,000,000		1,323,400,000
Total assets at year end	2,537,200,000				4,887,100,000					2,537,200,000	4,887,100,000		6,121,000,000
Exploration and production [Member] | Other [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Segment income (loss)										(158,600,000)	(250,000,000)		(373,800,000)
Revenues from external customers										0	(1,300,000)		83,600,000
Interest income										0	0		0
Interest expense										0	0		0
Income tax expense (benefit)										(17,300,000)	(95,900,000)		(120,800,000)
Depreciation, depletion and amortization										6,200,000	5,100,000		4,500,000
Accretion of asset retirement obligations										0	0		4,300,000
Amortization of undeveloped leases										1,800,000	4,900,000		15,600,000
Impairment of assets										0	0		0
Deferred and noncurrent income taxes										(4,600,000)	0		0
Additions to property, plant, equipment										39,200,000	10,700,000		64,800,000
Total assets at year end	$ 147,700,000				$ 138,700,000					$ 147,700,000	$ 138,700,000		$ 180,400,000

[1] Reclassified to conform to current presentation.