Business Segments (Segment Information) (Details) - USD ($)		3 Months Ended										12 Months Ended
		Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015		Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Dec. 31, 2016	Dec. 31, 2015		Dec. 31, 2014
Revenues From External Customers And Long Lived Assets [Line Items]
Net income (loss)		$ (63,900,000)	$ (16,200,000)	$ 2,900,000	$ (198,800,000)	$ (587,100,000)		$ (1,595,400,000)	$ (73,800,000)	$ (14,500,000)		$ (275,970,000)	$ (2,270,833,000)		$ 905,611,000
Revenues from external customers												1,874,129,000	3,033,080,000		5,476,084,000
Income tax expense (benefit)												(219,172,000)	(1,026,490,000)		227,297,000
Depreciation, depletion and amortization												1,054,081,000	1,619,824,000		1,906,247,000
Accretion of asset retirement obligations												46,742,000	48,665,000		50,778,000
Amortization of undeveloped leases												43,417,000	75,312,000		74,438,000
Impairment of assets												95,088,000	2,493,156,000		51,314,000
Total assets at year end		10,295,860,000				11,493,812,000	[1]					10,295,860,000	11,493,812,000	[1]
Discontinued Operations [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net income (loss)												(2,000,000)	(15,000,000)		(119,400,000)
Total assets at year end		27,100,000				38,300,000					$ 427,100,000	27,100,000	38,300,000		427,100,000
United States [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Revenues from external customers												693,200,000	1,260,000,000		2,201,500,000
Impairment of assets	[2]												329,000,000		14,300,000
Canada [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Revenues from external customers												421,100,000	557,300,000		1,052,400,000
Impairment of assets											37,047,000	95,088,000	683,574,000		37,047,000	[3]
Malaysia [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Revenues from external customers												759,300,000	1,210,900,000		2,233,000,000
Impairment of assets	[2]												1,480,600,000
Other [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Revenues from external customers												500,000	4,900,000		(10,800,000)
Operating Segments [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net income (loss)												(276,000,000)	(2,270,800,000)		905,600,000
Revenues from external customers												1,874,100,000	3,033,100,000
Interest income												2,900,000	4,000,000		7,700,000
Interest expense, net of capitalization												148,200,000	117,400,000		115,800,000
Income tax expense (benefit)												(219,200,000)	(1,026,500,000)		227,300,000
Depreciation, depletion and amortization												1,054,100,000	1,619,800,000		1,906,200,000
Accretion of asset retirement obligations												46,700,000	48,700,000		50,800,000
Amortization of undeveloped leases												43,400,000	75,400,000		74,400,000
Impairment of assets												95,100,000	2,493,200,000		51,300,000
Deferred and noncurrent income taxes												(387,800,000)	(978,000,000)		(170,900,000)
Additions to property, plant, equipment												753,100,000	1,791,500,000		3,317,800,000
Total assets at year end		10,295,900,000				11,493,800,000					16,742,300,000	10,295,900,000	11,493,800,000		16,742,300,000
Exploration and production [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net income (loss)												(124,900,000)	(2,010,900,000)		1,189,800,000
Revenues from external customers												1,804,600,000	2,934,700,000		5,422,700,000
Interest income												0	0		0
Interest expense, net of capitalization												0	0		0
Income tax expense (benefit)												(155,100,000)	(1,111,000,000)		285,700,000
Depreciation, depletion and amortization												1,037,300,000	1,607,900,000		1,897,500,000
Accretion of asset retirement obligations												46,700,000	48,700,000		50,800,000
Amortization of undeveloped leases												43,400,000	75,400,000		74,400,000
Impairment of assets												95,100,000	2,493,200,000		51,300,000
Deferred and noncurrent income taxes												(311,000,000)	(917,500,000)		(152,100,000)
Additions to property, plant, equipment												731,200,000	1,731,600,000		3,303,300,000
Total assets at year end		9,118,900,000				10,863,300,000					14,541,300,000	9,118,900,000	10,863,300,000		14,541,300,000
Exploration and production [Member] | United States [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net income (loss)												(205,400,000)	(615,700,000)		387,100,000
Revenues from external customers												685,700,000	1,253,600,000		2,196,400,000
Interest income												0	0		0
Interest expense, net of capitalization												0	0		0
Income tax expense (benefit)												(87,900,000)	(337,000,000)		214,800,000
Depreciation, depletion and amortization												600,500,000	794,900,000		840,700,000
Accretion of asset retirement obligations												17,100,000	20,200,000		17,500,000
Amortization of undeveloped leases												38,400,000	59,200,000		50,100,000
Impairment of assets												0	329,000,000		14,300,000
Deferred and noncurrent income taxes												(108,400,000)	(187,700,000)		39,700,000
Additions to property, plant, equipment												269,800,000	1,263,100,000		2,028,700,000
Total assets at year end		5,419,000,000				5,717,800,000					5,745,700,000	5,419,000,000	5,717,800,000		5,745,700,000
Exploration and production [Member] | Canada [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net income (loss)												(35,900,000)	(583,400,000)		156,500,000
Revenues from external customers												365,300,000	549,700,000		1,044,100,000
Interest income												0	0		0
Interest expense, net of capitalization												0	0		0
Income tax expense (benefit)												(134,300,000)	(188,800,000)		64,200,000
Depreciation, depletion and amortization												203,200,000	261,900,000		316,700,000
Accretion of asset retirement obligations												13,300,000	12,600,000		15,200,000
Amortization of undeveloped leases												4,500,000	14,400,000		19,400,000
Impairment of assets												95,100,000	683,600,000		37,000,000
Deferred and noncurrent income taxes												(175,800,000)	(146,000,000)		43,300,000
Additions to property, plant, equipment												361,300,000	184,900,000		445,900,000
Total assets at year end		1,559,500,000				2,460,600,000					3,769,800,000	1,559,500,000	2,460,600,000		3,769,800,000
Exploration and production [Member] | Malaysia [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net income (loss)												171,100,000	(653,200,000)		896,200,000
Revenues from external customers												753,400,000	1,131,400,000		2,183,500,000
Interest income												0	0		0
Interest expense, net of capitalization												0	0		0
Income tax expense (benefit)												85,900,000	(567,900,000)		102,600,000
Depreciation, depletion and amortization												227,700,000	544,900,000		735,000,000
Accretion of asset retirement obligations												16,300,000	15,900,000		18,100,000
Amortization of undeveloped leases												0	0		0
Impairment of assets												0	1,480,600,000		0
Deferred and noncurrent income taxes												(8,500,000)	(579,200,000)		(235,100,000)
Additions to property, plant, equipment												101,400,000	244,400,000		818,000,000
Total assets at year end		2,024,700,000				2,537,200,000					4,887,100,000	2,024,700,000	2,537,200,000		4,887,100,000
Exploration and production [Member] | Other [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net income (loss)												(54,700,000)	(158,600,000)		(250,000,000)
Revenues from external customers												200,000	0		(1,300,000)
Interest income												0	0		0
Interest expense, net of capitalization												0	0		0
Income tax expense (benefit)												(18,800,000)	(17,300,000)		(95,900,000)
Depreciation, depletion and amortization												5,900,000	6,200,000		5,100,000
Accretion of asset retirement obligations												0	0		0
Amortization of undeveloped leases												500,000	1,800,000		4,900,000
Impairment of assets												0	0		0
Deferred and noncurrent income taxes												(18,300,000)	(4,600,000)		0
Additions to property, plant, equipment												(1,300,000)	39,200,000		10,700,000
Total assets at year end		115,700,000				147,700,000					138,700,000	115,700,000	147,700,000		138,700,000
Corporate and Other [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net income (loss)												(149,100,000)	(244,900,000)		(164,800,000)
Revenues from external customers												69,500,000	98,400,000
Interest income												2,900,000	4,000,000		7,700,000
Interest expense, net of capitalization												148,200,000	117,400,000		115,800,000
Income tax expense (benefit)												(64,100,000)	84,500,000		(58,400,000)
Depreciation, depletion and amortization												16,800,000	11,900,000		8,700,000
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												(76,800,000)	(60,500,000)		(18,800,000)
Additions to property, plant, equipment												21,900,000	59,900,000		14,500,000
Total assets at year end		$ 1,149,900,000				$ 592,200,000					$ 1,773,900,000	$ 1,149,900,000	$ 592,200,000		$ 1,773,900,000

[1] Reclassified to conform to current presentation. See Note B for additional information.

[2] Results exclude corporate overhead, interest and discontinued operations.

[3] This amount represented the writeoff of goodwill associated with an oil and gas company acquired in 2000.