Business Segments - Segment Information (Details) - USD ($)	3 Months Ended									12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018		Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018		Dec. 31, 2017
Revenues from External Customers and Long-Lived Assets [Line Items]
Net income (loss) including noncontrolling interest	$ (54,400,000)	$ 1,111,700,000	$ 123,200,000	$ 72,800,000	$ 111,800,000		$ 93,900,000	$ 45,500,000	$ 168,300,000	$ 1,253,302,000	$ 419,486,000	[1],[2]	$ (311,789,000)	[1],[2]
Revenues from external customers										2,829,053,000	1,791,401,000	[1]	1,443,988,000	[1]
Interest and other income (loss)										(22,520,000)	7,774,000	[1]	(78,302,000)	[1]
Interest expense, net of capitalization										(219,275,000)	(180,359,000)	[1]	(178,263,000)	[1]
Income tax expense (benefit)										14,683,000	(126,136,000)	[1]	270,131,000	[1]
Depreciation, depletion and amortization										1,147,842,000	775,614,000	[1],[2]	751,878,000	[1],[2]
Accretion of asset retirement obligations										40,506,000	27,119,000	[1],[2]	25,282,000	[1],[2]
Deferred income tax charge (benefit)										28,000,000.0	40,100,000		61,800,000
Impairment of assets										0	20,000,000	[1],[2]	0	[1],[2]
Deferred and noncurrent income taxes										28,500,000	(183,200,000)		264,100,000
Additions to property, plant, equipment										2,560,800,000	1,894,500,000		870,800,000
Total assets at year-end	11,718,504,000				11,052,587,000	[2]				11,718,504,000	11,052,587,000	[2]	9,860,900,000
Mark to market of crude contracts										(227,000)	(54,000)	[2]	(127,434,000)	[2]
United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues from external customers										2,370,400,000	1,300,300,000		958,300,000
Canada
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues from external customers										447,100,000	468,900,000		485,700,000
Other
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues from external customers										11,600,000	22,200,000		0
Discontinued Operations
Revenues from External Customers and Long-Lived Assets [Line Items]
Net income (loss) including noncontrolling interest										1,064,500,000	250,300,000		241,200,000
Revenues from external customers										0	0		0
Interest and other income (loss)										0	0		0
Interest expense, net of capitalization										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
Deferred income tax charge (benefit)										0	0		0
Deferred and noncurrent income taxes										0	0		0
Additions to property, plant, equipment										0	138,600,000		16,000,000.0
Total assets at year-end	16,700,000				1,691,200,000					16,700,000	1,691,200,000		1,693,000,000.0
Operating Segments
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues from external customers										2,829,100,000	1,791,400,000		1,444,000,000.0
Exploration and Production
Revenues from External Customers and Long-Lived Assets [Line Items]
Net income (loss) including noncontrolling interest										460,600,000	277,400,000		66,100,000
Revenues from external customers										2,825,600,000	1,825,400,000		1,429,800,000
Interest and other income (loss)										(15,800,000)	0		0
Interest expense, net of capitalization										(500,000)	200,000		0
Income tax expense (benefit)										100,300,000	57,300,000		7,400,000
Depreciation, depletion and amortization										1,125,200,000	755,400,000		735,300,000
Accretion of asset retirement obligations										40,500,000	27,100,000		25,300,000
Deferred income tax charge (benefit)										28,000,000.0	40,100,000		61,800,000
Deferred and noncurrent income taxes										112,400,000	58,900,000		21,600,000
Additions to property, plant, equipment										2,547,200,000	1,733,200,000		840,000,000.0
Total assets at year-end	10,655,600,000				8,242,900,000					10,655,600,000	8,242,900,000		7,066,200,000
Exploration and Production | United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Net income (loss) including noncontrolling interest										518,400,000	242,900,000		(8,900,000)
Revenues from external customers										2,367,000,000.0	1,332,700,000		944,300,000
Interest and other income (loss)										(13,400,000)	0		0
Interest expense, net of capitalization										0	0		0
Income tax expense (benefit)										115,600,000	68,100,000		(800,000)
Depreciation, depletion and amortization										878,700,000	519,500,000		546,100,000
Accretion of asset retirement obligations										34,400,000	19,500,000		17,400,000
Deferred income tax charge (benefit)										23,100,000	36,800,000		60,200,000
Deferred and noncurrent income taxes										111,800,000	68,100,000		2,500,000
Additions to property, plant, equipment										2,193,300,000	1,343,500,000		534,800,000
Total assets at year-end	8,043,300,000				6,342,900,000					8,043,300,000	6,342,900,000		5,186,200,000
Exploration and Production | Canada
Revenues from External Customers and Long-Lived Assets [Line Items]
Net income (loss) including noncontrolling interest										(4,300,000)	51,100,000		112,500,000
Revenues from external customers										447,000,000.0	470,500,000		485,500,000
Interest and other income (loss)										(1,500,000)	0		0
Interest expense, net of capitalization										(100,000)	0		0
Income tax expense (benefit)										(2,900,000)	14,500,000		44,400,000
Depreciation, depletion and amortization										243,000,000.0	232,400,000		185,400,000
Accretion of asset retirement obligations										6,100,000	7,600,000		7,900,000
Deferred income tax charge (benefit)										1,300,000	800,000		1,600,000
Deferred and noncurrent income taxes										14,000,000.0	16,500,000		55,300,000
Additions to property, plant, equipment										284,100,000	373,800,000		267,600,000
Total assets at year-end	2,303,700,000				1,711,900,000					2,303,700,000	1,711,900,000		1,725,800,000
Exploration and Production | Other
Revenues from External Customers and Long-Lived Assets [Line Items]
Net income (loss) including noncontrolling interest										(53,500,000)	(16,600,000)		(37,500,000)
Revenues from external customers										11,600,000	22,200,000		0
Interest and other income (loss)										(900,000)	0		0
Interest expense, net of capitalization										(400,000)	200,000		0
Income tax expense (benefit)										(12,400,000)	(25,300,000)		(36,200,000)
Depreciation, depletion and amortization										3,500,000	3,500,000		3,800,000
Accretion of asset retirement obligations										0	0		0
Deferred income tax charge (benefit)										3,600,000	2,500,000		0
Deferred and noncurrent income taxes										(13,400,000)	(25,700,000)		(36,200,000)
Additions to property, plant, equipment										69,800,000	15,900,000		37,600,000
Total assets at year-end	308,600,000				188,100,000					308,600,000	188,100,000		154,200,000
Corporate and other
Revenues from External Customers and Long-Lived Assets [Line Items]
Net income (loss) including noncontrolling interest										(271,800,000)	(108,200,000)		(619,100,000)
Revenues from external customers										3,500,000	(34,000,000.0)		14,200,000
Interest and other income (loss)										(6,700,000)	7,800,000		(78,300,000)
Interest expense, net of capitalization										(218,800,000)	(180,600,000)		(178,300,000)
Income tax expense (benefit)										(85,600,000)	(183,400,000)		262,700,000
Depreciation, depletion and amortization										22,600,000	20,200,000		16,600,000
Accretion of asset retirement obligations										0	0		0
Deferred income tax charge (benefit)										0	0		0
Deferred and noncurrent income taxes										(83,900,000)	(242,100,000)		242,500,000
Additions to property, plant, equipment										13,600,000	22,700,000		14,800,000
Total assets at year-end	$ 1,046,200,000				$ 1,118,500,000					$ 1,046,200,000	$ 1,118,500,000		$ 1,101,700,000

[1] ¹ Reclassified to conform with current presentation (see Notes C and E). 2017 revenue is presented net of $58.9 million of transportation, gathering and processing costs as historically shown prior to the adoption of ASC 606 on January 1, 2018. See Notes to Consolidated Financial Statements, page 58.

[2] Reclassified to conform with current presentation (see Note E)