Business Segments (Segment Information) (Details) - USD ($)		3 Months Ended								12 Months Ended
		Dec. 31, 2018		Sep. 30, 2018	[1]	Jun. 30, 2018	[1]	Mar. 31, 2018	[1]	Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016
Revenues From External Customers And Long Lived Assets [Line Items]
Net income (loss) including noncontrolling interest		$ 111,800,000	[1]	$ 93,900,000		$ 45,500,000		$ 168,300,000		$ 419,486,000		$ (311,789,000)	[2]	$ (275,970,000)	[2]
Revenues from external customers										2,570,603,000		2,225,129,000	[2]	1,811,238,000	[2]
Interest expense, net of capitalization										181,604,000		181,783,000	[2]	148,170,000	[2]
Income tax expense (benefit)										9,330,000		382,738,000	[2]	(219,172,000)	[2]
Depreciation, depletion and amortization										971,901,000		957,719,000	[2]	1,054,081,000	[2]
Accretion of asset retirement obligations										44,559,000		42,590,000	[2]	46,742,000	[2]
Amortization of undeveloped leases										40,177,000		61,776,000		43,417,000
Impairment of assets										20,000,000		0		95,088,000	[2]
Total assets at year end		11,052,587,000								11,052,587,000		9,860,942,000
United States [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Revenues from external customers										1,297,500,000		958,300,000		692,300,000
Canada [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Revenues from external customers										438,500,000		485,700,000		365,300,000
Impairment of assets														95,088,000
Malaysia [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Revenues from external customers										854,300,000		781,100,000		753,400,000
Other [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Revenues from external customers										(19,700,000)				200,000
Discontinued Operations [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net income (loss) including noncontrolling interest										(3,500,000)		(900,000)		(2,000,000)
Total assets at year end		20,900,000								20,900,000		22,900,000		27,100,000
Operating Segments [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net income (loss) including noncontrolling interest										419,500,000		(311,800,000)		(276,000,000)
Revenues from external customers										2,570,600,000		2,225,100,000		1,811,200,000
Interest income										8,000,000		7,400,000		2,900,000
Interest expense, net of capitalization										181,600,000		181,800,000		148,200,000
Income tax expense (benefit)										9,300,000		382,700,000		(219,200,000)
Depreciation, depletion and amortization										971,900,000		957,700,000		1,054,100,000
Accretion of asset retirement obligations										44,600,000		42,600,000		46,700,000
Amortization of undeveloped leases										40,100,000		61,800,000		43,400,000
Impairment of assets														95,100,000
Deferred and noncurrent income taxes										(183,700,000)		260,400,000		(387,800,000)
Additions to property, plant, equipment										1,894,500,000		870,800,000		753,100,000
Total assets at year end		11,052,500,000								11,052,500,000		9,860,900,000		10,295,900,000
Exploration And Production Segment [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net income (loss) including noncontrolling interest										546,900,000		290,300,000		(83,700,000)
Revenues from external customers										2,604,600,000	[3]	2,210,900,000		1,868,000,000
Interest income
Interest expense, net of capitalization										200,000
Income tax expense (benefit)										200,600,000		133,800,000		(132,900,000)
Depreciation, depletion and amortization										954,000,000		939,900,000		1,037,300,000
Accretion of asset retirement obligations										44,600,000		42,600,000		46,700,000
Amortization of undeveloped leases										40,100,000		61,800,000		43,400,000
Impairment of assets										20,000,000				95,100,000
Deferred and noncurrent income taxes										58,400,000		17,900,000		(311,000,000)
Additions to property, plant, equipment										1,871,800,000		856,000,000		731,200,000
Total assets at year end		9,913,100,000								9,913,100,000		8,736,300,000		9,118,900,000
Exploration And Production Segment [Member] | United States [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net income (loss) including noncontrolling interest										242,900,000		(8,900,000)		(164,200,000)
Revenues from external customers										1,289,600,000	[3]	944,300,000		749,100,000
Interest income
Interest expense, net of capitalization
Income tax expense (benefit)										68,100,000		(800,000)		(65,700,000)
Depreciation, depletion and amortization										519,500,000		546,100,000		600,500,000
Accretion of asset retirement obligations										19,500,000		17,400,000		17,100,000
Amortization of undeveloped leases										36,800,000		60,200,000		38,400,000
Impairment of assets										20,000,000
Deferred and noncurrent income taxes										68,100,000		2,500,000		(108,400,000)
Additions to property, plant, equipment										1,343,500,000		534,800,000		269,800,000
Total assets at year end		6,342,900,000								6,342,900,000		5,186,200,000		5,419,000,000
Exploration And Production Segment [Member] | Canada [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net income (loss) including noncontrolling interest	[4]									51,100,000		112,500,000		(35,900,000)
Revenues from external customers	[4]									438,600,000	[3]	485,500,000		365,300,000
Interest income	[4]
Interest expense, net of capitalization	[4]
Income tax expense (benefit)	[4]									14,500,000		44,400,000		(134,300,000)
Depreciation, depletion and amortization	[4]									232,400,000		185,400,000		203,200,000
Accretion of asset retirement obligations	[4]									7,700,000		7,900,000		13,300,000
Amortization of undeveloped leases	[4]									800,000		1,600,000		4,500,000
Impairment of assets	[4]													95,100,000
Deferred and noncurrent income taxes	[4]									16,500,000		55,300,000		(175,800,000)
Additions to property, plant, equipment	[4]									373,800,000		267,600,000		361,300,000
Total assets at year end	[4]	1,711,900,000								1,711,900,000		1,725,800,000		1,559,500,000
Exploration And Production Segment [Member] | Malaysia [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net income (loss) including noncontrolling interest										269,500,000		224,200,000		171,100,000
Revenues from external customers										854,200,000	[3]	781,100,000		753,400,000
Interest income
Interest expense, net of capitalization
Income tax expense (benefit)										143,300,000		126,400,000		85,900,000
Depreciation, depletion and amortization										198,600,000		204,600,000		227,700,000
Accretion of asset retirement obligations										17,400,000		17,300,000		16,300,000
Deferred and noncurrent income taxes										(500,000)		(3,700,000)		(8,500,000)
Additions to property, plant, equipment										138,600,000		16,000,000		101,400,000
Total assets at year end		1,670,200,000								1,670,200,000		1,670,100,000		2,024,700,000
Exploration And Production Segment [Member] | Other [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net income (loss) including noncontrolling interest										(16,600,000)		(37,500,000)		(54,700,000)
Revenues from external customers										22,200,000	[3]			200,000
Interest income
Interest expense, net of capitalization										200,000
Income tax expense (benefit)										(25,300,000)		(36,200,000)		(18,800,000)
Depreciation, depletion and amortization										3,500,000		3,800,000		5,900,000
Amortization of undeveloped leases										2,500,000				500,000
Deferred and noncurrent income taxes										(25,700,000)		(36,200,000)		(18,300,000)
Additions to property, plant, equipment										15,900,000		37,600,000		(1,300,000)
Total assets at year end		188,100,000								188,100,000		154,200,000		115,700,000
Corporate and Other [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net income (loss) including noncontrolling interest										(123,900,000)		(607,500,000)		(149,100,000)
Revenues from external customers										(34,000,000)		4,600,000		6,600,000
Interest income										8,000,000		7,400,000		2,900,000
Interest expense, net of capitalization										181,400,000		181,800,000		148,200,000
Income tax expense (benefit)										(191,300,000)		245,600,000		(64,100,000)
Depreciation, depletion and amortization										17,900,000		17,800,000		16,800,000
Accretion of asset retirement obligations										0		0		0
Amortization of undeveloped leases										0		0		0
Impairment of assets														0
Deferred and noncurrent income taxes										(242,100,000)		242,500,000		(76,800,000)
Additions to property, plant, equipment										22,700,000		14,800,000		21,900,000
Total assets at year end		$ 1,118,500,000								$ 1,118,500,000		$ 1,101,700,000		$ 1,149,900,000

[1] 2018 includes noncontrolling interest in MP GOM.

[2] Reclassified to conform to current presentation (see Note B).

[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. This business was sold in June 2016.