Business Segments (Segment Information) (Details) (USD $)	3 Months Ended										12 Months Ended
	Dec. 31, 2014		Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013		Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Segment Reporting Information [Line Items]
Net income for the year	$ 375,200,000		$ 245,700,000	$ 129,400,000	$ 155,300,000	$ 75,400,000		$ 284,800,000	$ 402,700,000	$ 360,600,000	$ 905,611,000		$ 1,123,473,000		$ 970,876,000
External Revenues											5,476,084,000		5,390,089,000		4,619,602,000
Income tax expense (benefit)											227,297,000		584,550,000		561,516,000
Significant noncash charges (credits)
Depreciation, depletion, amortization											1,906,247,000		1,553,394,000		1,253,095,000
Accretion of asset retirement obligations											50,778,000		48,996,000		38,361,000
Amortization of undeveloped leases											74,438,000		66,891,000		129,750,000
Impairment of assets											51,314,000		21,587,000		200,000,000	[1]
Total Assets	16,742,307,000					17,509,484,000					16,742,307,000		17,509,484,000
External Customer [Member]
Segment Reporting Information [Line Items]
External Revenues											5,476,100,000		5,390,100,000		4,619,600,000
United States [Member]
Segment Reporting Information [Line Items]
External Revenues											2,201,500,000		1,798,500,000		1,038,100,000
Canada [Member]
Segment Reporting Information [Line Items]
External Revenues											1,052,400,000		1,150,200,000		1,088,400,000
Significant noncash charges (credits)
Impairment of assets	37,047,000
Malaysia [Member]
Segment Reporting Information [Line Items]
External Revenues											2,233,000,000		2,337,500,000		2,415,600,000
Other [Member]
Segment Reporting Information [Line Items]
External Revenues											(10,800,000)		103,900,000		77,500,000
Discontinued Operations [Member]
Segment Reporting Information [Line Items]
Net income for the year											(119,400,000)		235,400,000		164,400,000
Interest income											0		0		0
Interest expense, net of capitalization											0		0		0
Income tax expense (benefit)											0		0		0
Significant noncash charges (credits)
Depreciation, depletion, 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		8,100,000		191,800,000
Total Assets	427,100,000					1,325,100,000					427,100,000		1,325,100,000		3,410,100,000
Discontinued Operations [Member] | External Customer [Member]
Segment Reporting Information [Line Items]
External Revenues											0		0		0
Operating Segments [Member]
Segment Reporting Information [Line Items]
Net income for the year											905,600,000		1,123,500,000		970,900,000
Interest income											7,700,000		3,900,000		6,500,000
Interest expense, net of capitalization											115,800,000		71,900,000		14,900,000
Income tax expense (benefit)											227,300,000		584,600,000		561,500,000
Significant noncash charges (credits)
Depreciation, depletion, amortization											1,906,200,000		1,553,400,000		1,253,100,000
Accretion of asset retirement obligations											50,800,000		49,000,000		38,400,000
Amortization of undeveloped leases											74,400,000		66,900,000		129,800,000
Impairment of assets											51,300,000		21,600,000		200,000,000
Deferred and noncurrent income taxes											(170,900,000)		158,100,000		342,800,000
Additions to property, plant, equipment											3,317,800,000		3,532,200,000		4,133,800,000
Total Assets	16,742,300,000					17,509,500,000					16,742,300,000		17,509,500,000		17,522,600,000
Operating Segments [Member] | External Customer [Member]
Segment Reporting Information [Line Items]
External Revenues											5,476,100,000		5,390,100,000		4,619,600,000
Exploration and production [Member]
Segment Reporting Information [Line Items]
Net income for the year											1,189,800,000		1,028,800,000		905,000,000
Interest income											0		0		0
Interest expense, net of capitalization											0		0		0
Income tax expense (benefit)											285,700,000		656,300,000		605,000,000
Significant noncash charges (credits)
Depreciation, depletion, amortization											1,897,500,000		1,543,600,000		1,244,400,000
Accretion of asset retirement obligations											50,800,000		49,000,000		38,400,000
Amortization of undeveloped leases											74,400,000		66,900,000		129,800,000
Impairment of assets											51,300,000		21,600,000		200,000,000
Deferred and noncurrent income taxes											(152,100,000)		173,800,000		375,100,000
Additions to property, plant, equipment											3,303,300,000		3,508,600,000		3,933,800,000
Total Assets	14,541,300,000					14,919,200,000					14,541,300,000		14,919,200,000		13,102,900,000
Exploration and production [Member] | External Customer [Member]
Segment Reporting Information [Line Items]
External Revenues											5,422,700,000		5,312,600,000		4,608,100,000
Exploration and production [Member] | United States [Member]
Segment Reporting Information [Line Items]
Net income for the year											387,100,000		435,400,000		168,000,000
Interest income											0		0		0
Interest expense, net of capitalization											0		0		0
Income tax expense (benefit)											214,800,000		241,600,000		99,800,000
Significant noncash charges (credits)
Depreciation, depletion, amortization											840,700,000		576,300,000		330,200,000
Accretion of asset retirement obligations											17,500,000		13,500,000		11,400,000
Amortization of undeveloped leases											50,100,000		30,300,000		71,600,000
Impairment of assets											14,300,000		0		0
Deferred and noncurrent income taxes											39,700,000		99,600,000		231,000,000
Additions to property, plant, equipment											2,028,700,000		1,785,900,000		1,615,900,000
Total Assets	5,745,700,000					4,530,000,000					5,745,700,000		4,530,000,000		3,625,900,000
Exploration and production [Member] | United States [Member] | External Customer [Member]
Segment Reporting Information [Line Items]
External Revenues											2,196,400,000		1,803,800,000		1,038,000,000
Exploration and production [Member] | Canada [Member]
Segment Reporting Information [Line Items]
Net income for the year											156,500,000		180,800,000		208,100,000
Interest income											0		0		0
Interest expense, net of capitalization											0		0		0
Income tax expense (benefit)											64,200,000		57,800,000		65,100,000
Significant noncash charges (credits)
Depreciation, depletion, amortization											316,700,000		374,600,000		345,800,000
Accretion of asset retirement obligations											15,200,000		16,200,000		13,600,000
Amortization of undeveloped leases											19,400,000		21,000,000		29,300,000
Impairment of assets											37,000,000		21,600,000		0
Deferred and noncurrent income taxes											43,300,000		26,100,000		72,300,000
Additions to property, plant, equipment											445,900,000		334,500,000		887,200,000
Total Assets	3,769,800,000					4,087,800,000					3,769,800,000		4,087,800,000		4,477,700,000
Exploration and production [Member] | Canada [Member] | External Customer [Member]
Segment Reporting Information [Line Items]
External Revenues											1,044,100,000		1,144,700,000		1,084,300,000
Exploration and production [Member] | Malaysia [Member]
Segment Reporting Information [Line Items]
Net income for the year											896,200,000		786,400,000		894,200,000
Interest income											0		0		0
Interest expense, net of capitalization											0		0		0
Income tax expense (benefit)											102,600,000		477,700,000		544,700,000
Significant noncash charges (credits)
Depreciation, depletion, amortization											735,000,000		588,200,000		532,100,000
Accretion of asset retirement obligations											18,100,000		15,000,000		12,500,000
Amortization of undeveloped leases											0		0		0
Impairment of assets											0		0		0
Deferred and noncurrent income taxes											(235,100,000)		48,100,000		73,300,000
Additions to property, plant, equipment											818,000,000		1,323,400,000		1,426,700,000
Total Assets	4,887,100,000					6,121,000,000					4,887,100,000		6,121,000,000		4,811,500,000
Exploration and production [Member] | Malaysia [Member] | External Customer [Member]
Segment Reporting Information [Line Items]
External Revenues											2,183,500,000		2,280,500,000		2,428,100,000
Exploration and production [Member] | Other [Member]
Segment Reporting Information [Line Items]
Net income for the year											(250,000,000)	[2]	(373,800,000)	[2]	(365,300,000)	[2]
Interest income											0	[2]	0	[2]	0	[2]
Interest expense, net of capitalization											0	[2]	0	[2]	0	[2]
Income tax expense (benefit)											(95,900,000)	[2]	(120,800,000)	[2]	(104,600,000)	[2]
Significant noncash charges (credits)
Depreciation, depletion, amortization											5,100,000	[2]	4,500,000	[2]	36,300,000	[2]
Accretion of asset retirement obligations											0	[2]	4,300,000	[2]	900,000	[2]
Amortization of undeveloped leases											4,900,000	[2]	15,600,000	[2]	28,900,000	[2]
Impairment of assets											0	[2]	0	[2]	200,000,000	[2]
Deferred and noncurrent income taxes											0	[2]	0	[2]	(1,500,000)	[2]
Additions to property, plant, equipment											10,700,000	[2]	64,800,000	[2]	4,000,000	[2]
Total Assets	138,700,000	[2]				180,400,000	[2]				138,700,000	[2]	180,400,000	[2]	187,800,000	[2]
Exploration and production [Member] | Other [Member] | External Customer [Member]
Segment Reporting Information [Line Items]
External Revenues											(1,300,000)	[2]	83,600,000	[2]	57,700,000	[2]
Corporate and Other [Member]
Segment Reporting Information [Line Items]
Net income for the year											(164,800,000)		(140,700,000)		(98,500,000)
Interest income											7,700,000		3,900,000		6,500,000
Interest expense, net of capitalization											115,800,000		71,900,000		14,900,000
Income tax expense (benefit)											(58,400,000)		(71,700,000)		(43,500,000)
Significant noncash charges (credits)
Depreciation, depletion, amortization											8,700,000		9,800,000		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											(18,800,000)		(15,700,000)		(32,300,000)
Additions to property, plant, equipment											14,500,000		15,500,000		8,200,000
Total Assets	1,773,900,000					1,265,200,000					1,773,900,000		1,265,200,000		1,009,600,000
Corporate and Other [Member] | External Customer [Member]
Segment Reporting Information [Line Items]
External Revenues											$ 53,400,000		$ 77,500,000		$ 11,500,000

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

[2] Reclassified to conform to current presentation.