Business Segments (Segment Information) (Details) - USD ($)		3 Months Ended								12 Months Ended
		Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Revenues From External Customers And Long Lived Assets [Line Items]
Net income (loss)		$ (286,800,000)	$ (65,900,000)	$ (17,600,000)	$ 58,500,000	$ (63,900,000)	$ (16,200,000)	$ 2,900,000	$ (198,800,000)	$ (311,789,000)		$ (275,970,000)	[1]	$ (2,270,833,000)	[1]
Revenues from external customers										2,225,129,000		1,811,238,000	[1]	2,941,271,000	[1]
Interest expense, net of capitalization										181,783,000		148,170,000	[1]	117,375,000	[1]
Income tax expense (benefit)										382,738,000		(219,172,000)	[1]	(1,026,490,000)	[1]
Depreciation, depletion and amortization										957,719,000		1,054,081,000	[1]	1,619,824,000	[1]
Accretion of asset retirement obligations										42,590,000		46,742,000	[1]	48,665,000	[1]
Amortization of undeveloped leases										61,776,000		43,417,000		75,312,000
Impairment of assets										0		95,088,000	[1]	2,493,156,000	[1]
Total assets at year end		9,860,942,000				10,295,860,000				9,860,942,000		10,295,860,000
United States [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Revenues from external customers										958,300,000		692,300,000		1,260,200,000
Impairment of assets	[2]													329,000,000
Canada [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Revenues from external customers										485,700,000		365,300,000		549,700,000
Impairment of assets												95,088,000		683,574,000
Malaysia [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Revenues from external customers										781,100,000		753,400,000		1,131,400,000
Impairment of assets	[2]													1,480,600,000
Other [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Revenues from external customers												200,000
Discontinued Operations [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net income (loss)										(900,000)		(2,000,000)		(15,000,000)
Total assets at year end		22,900,000				27,100,000				22,900,000		27,100,000		38,300,000
Operating Segments [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net income (loss)										(311,800,000)		(276,000,000)		(2,270,800,000)
Revenues from external customers										2,225,100,000		1,811,200,000		2,941,300,000
Interest income										7,400,000		2,900,000		4,000,000
Interest expense, net of capitalization										181,800,000		148,200,000		117,400,000
Income tax expense (benefit)										382,700,000		(219,200,000)		(1,026,500,000)
Depreciation, depletion and amortization										957,700,000		1,054,100,000		1,619,800,000
Accretion of asset retirement obligations										42,600,000		46,700,000		48,700,000
Amortization of undeveloped leases										61,800,000		43,400,000		75,400,000
Impairment of assets												95,100,000		2,493,200,000
Deferred and noncurrent income taxes										260,400,000		(387,800,000)		(978,000,000)
Additions to property, plant, equipment										870,800,000		753,100,000		1,791,500,000
Total assets at year end		9,860,900,000				10,295,900,000				9,860,900,000		10,295,900,000		11,493,800,000
Exploration and production [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net income (loss)										296,600,000		(124,900,000)		(2,010,900,000)
Revenues from external customers										2,220,500,000	[3]	1,804,600,000		2,934,700,000
Interest income
Interest expense, net of capitalization
Income tax expense (benefit)										137,100,000		(155,100,000)		(1,111,000,000)
Depreciation, depletion and amortization										939,900,000		1,037,300,000		1,607,900,000
Accretion of asset retirement obligations										42,600,000		46,700,000		48,700,000
Amortization of undeveloped leases										61,800,000		43,400,000		75,400,000
Impairment of assets												95,100,000		2,493,200,000
Deferred and noncurrent income taxes										17,900,000		(311,000,000)		(917,500,000)
Additions to property, plant, equipment										856,000,000		731,200,000		1,731,600,000
Total assets at year end		8,736,300,000				9,118,900,000				8,736,300,000		9,118,900,000		10,863,300,000
Exploration and production [Member] | United States [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net income (loss)										(2,600,000)		(205,400,000)		(615,700,000)
Revenues from external customers										953,900,000	[3]	685,700,000		1,253,600,000
Interest income
Interest expense, net of capitalization
Income tax expense (benefit)										2,500,000		(87,900,000)		(337,000,000)
Depreciation, depletion and amortization										546,100,000		600,500,000		794,900,000
Accretion of asset retirement obligations										17,400,000		17,100,000		20,200,000
Amortization of undeveloped leases										60,200,000		38,400,000		59,200,000
Impairment of assets														329,000,000
Deferred and noncurrent income taxes										2,500,000		(108,400,000)		(187,700,000)
Additions to property, plant, equipment										534,800,000		269,800,000		1,263,100,000
Total assets at year end		5,186,200,000				5,419,000,000				5,186,200,000		5,419,000,000		5,717,800,000
Exploration and production [Member] | Canada [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net income (loss)	[4]									112,500,000		(35,900,000)		(583,400,000)
Revenues from external customers	[4]									485,500,000	[3]	365,300,000		549,700,000
Interest income	[4]
Interest expense, net of capitalization	[4]
Income tax expense (benefit)	[4]									44,400,000		(134,300,000)		(188,800,000)
Depreciation, depletion and amortization	[4]									185,400,000		203,200,000		261,900,000
Accretion of asset retirement obligations	[4]									7,900,000		13,300,000		12,600,000
Amortization of undeveloped leases	[4]									1,600,000		4,500,000		14,400,000
Impairment of assets	[4]											95,100,000		683,600,000
Deferred and noncurrent income taxes	[4]									55,300,000		(175,800,000)		(146,000,000)
Additions to property, plant, equipment	[4]									267,600,000		361,300,000		184,900,000
Total assets at year end	[4]	1,725,800,000				1,559,500,000				1,725,800,000		1,559,500,000		2,460,600,000
Exploration and production [Member] | Malaysia [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net income (loss)										224,200,000		171,100,000		(653,200,000)
Revenues from external customers										781,100,000	[3]	753,400,000		1,131,400,000
Interest income
Interest expense, net of capitalization
Income tax expense (benefit)										126,400,000		85,900,000		(567,900,000)
Depreciation, depletion and amortization										204,600,000		227,700,000		544,900,000
Accretion of asset retirement obligations										17,300,000		16,300,000		15,900,000
Impairment of assets														1,480,600,000
Deferred and noncurrent income taxes										(3,700,000)		(8,500,000)		(579,200,000)
Additions to property, plant, equipment										16,000,000		101,400,000		244,400,000
Total assets at year end		1,670,100,000				2,024,700,000				1,670,100,000		2,024,700,000		2,537,200,000
Exploration and production [Member] | Other [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net income (loss)										(37,500,000)		(54,700,000)		(158,600,000)
Revenues from external customers												200,000
Interest income
Interest expense, net of capitalization
Income tax expense (benefit)										(36,200,000)		(18,800,000)		(17,300,000)
Depreciation, depletion and amortization										3,800,000		5,900,000		6,200,000
Amortization of undeveloped leases												500,000		1,800,000
Deferred and noncurrent income taxes										(36,200,000)		(18,300,000)		(4,600,000)
Additions to property, plant, equipment										37,600,000		(1,300,000)		39,200,000
Total assets at year end		154,200,000				115,700,000				154,200,000		115,700,000		147,700,000
Corporate and Other [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net income (loss)										(607,500,000)		(149,100,000)		(244,900,000)
Revenues from external customers										4,600,000		6,600,000		6,600,000
Interest income										7,400,000		2,900,000		4,000,000
Interest expense, net of capitalization										181,800,000		148,200,000		117,400,000
Income tax expense (benefit)										245,600,000		(64,100,000)		84,500,000
Depreciation, depletion and amortization										17,800,000		16,800,000		11,900,000
Accretion of asset retirement obligations										0		0		0
Amortization of undeveloped leases										0		0		0
Impairment of assets												0		0
Deferred and noncurrent income taxes										242,500,000		(76,800,000)		(60,500,000)
Additions to property, plant, equipment										14,800,000		21,900,000		59,900,000
Total assets at year end		$ 1,101,700,000				$ 1,149,900,000				$ 1,101,700,000		$ 1,149,900,000		$ 592,200,000

[1] Reclassified to conform to current presentation (see Note A).

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

[3] Includes a pretax gain of $129.0 million on sale of Seal area heavy oil field sold in January 2017.

[4] Includes Synthetic crude operations in 2016 and 2015. This business was sold in June 2016.