Business Segments - Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenues from External Customers and Long-Lived Assets [Line Items]
Net income (loss) including noncontrolling interest	$ (162,700)	$ (266,600)	$ (324,400)	$ (508,700)	$ (54,400)	$ 1,111,700	$ 123,200	$ 72,800	$ (1,262,445)	$ 1,253,302	$ 419,486	[1]
Revenues from external customers									1,967,341	2,829,053	1,791,401
Interest and other income (loss)									(17,303)	(22,520)	7,774
Interest expense, net of capitalization									(169,423)	(219,275)	(180,359)
Income tax (benefit) expense									(293,741)	14,683	(126,136)
Impairment of assets									1,206,284	0	20,000	[1]
Depreciation, depletion and amortization									987,239	1,147,842	775,614	[1]
Accretion of asset retirement obligations									42,136	40,506	27,119	[1]
Amortization of undeveloped leases									26,743	27,973	40,177	[1]
Deferred income tax charge (benefit)									(278,042)	28,530	(177,627)	[1]
Additions to property, plant, equipment									756,600	2,560,800	1,894,500
Total assets at year-end	10,620,852				11,718,504				10,620,852	11,718,504	11,052,600
Previously Reported
Revenues from External Customers and Long-Lived Assets [Line Items]
Amortization of undeveloped leases											40,100
Deferred income tax charge (benefit)											(183,200)
UNITED STATES
Revenues from External Customers and Long-Lived Assets [Line Items]
Impairment of assets											20,000
Discontinued Operations
Revenues from External Customers and Long-Lived Assets [Line Items]
Net income (loss) including noncontrolling interest									(7,200)	1,064,500	250,300
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 (benefit) expense									0	0	0
Impairment of assets									0
Depreciation, depletion and amortization									0	0	0
Accretion of asset retirement obligations									0	0	0
Amortization of undeveloped leases									0	0
Deferred income tax charge (benefit)									0	0
Additions to property, plant, equipment									0	0	138,600
Total assets at year-end	700				16,700				700	16,700	1,691,200
Discontinued Operations | Previously Reported
Revenues from External Customers and Long-Lived Assets [Line Items]
Amortization of undeveloped leases											0
Deferred income tax charge (benefit)											0
Corporate and Other
Revenues from External Customers and Long-Lived Assets [Line Items]
Net income (loss) including noncontrolling interest									(120,300)	(271,800)	(108,200)
Revenues from external customers									207,900	3,500	(34,000)
Interest and other income (loss)									(9,100)	(6,700)	7,800
Interest expense, net of capitalization									(168,500)	(218,800)	(180,600)
Income tax (benefit) expense									(30,200)	(85,600)	(183,400)
Impairment of assets									14,100
Depreciation, depletion and amortization									22,300	22,600	20,200
Accretion of asset retirement obligations									0	0	0
Amortization of undeveloped leases									0	0
Deferred income tax charge (benefit)									(25,100)	(83,900)
Additions to property, plant, equipment									0	13,600	22,700
Total assets at year-end	1,032,900				1,046,200				1,032,900	1,046,200	1,118,500
Corporate and Other | Previously Reported
Revenues from External Customers and Long-Lived Assets [Line Items]
Amortization of undeveloped leases											0
Deferred income tax charge (benefit)											(242,100)
Exploration and Production | Operating Segments
Revenues from External Customers and Long-Lived Assets [Line Items]
Net income (loss) including noncontrolling interest									(1,134,900)	460,600	277,400
Revenues from external customers									1,759,400	2,825,600	1,825,400
Interest and other income (loss)									(8,200)	(15,800)	0
Interest expense, net of capitalization									(900)	(500)	200
Income tax (benefit) expense									(263,500)	100,300	57,300
Impairment of assets									1,192,200
Depreciation, depletion and amortization									964,900	1,125,200	755,400
Accretion of asset retirement obligations									42,100	40,500	27,100
Amortization of undeveloped leases									26,700	28,000
Deferred income tax charge (benefit)									(252,900)	112,400
Additions to property, plant, equipment									756,600	2,547,200	1,733,200
Total assets at year-end	9,587,300				10,655,600				9,587,300	10,655,600	8,242,900
Exploration and Production | Operating Segments | Previously Reported
Revenues from External Customers and Long-Lived Assets [Line Items]
Amortization of undeveloped leases											40,100
Deferred income tax charge (benefit)											58,900
Exploration and Production | Operating Segments | UNITED STATES
Revenues from External Customers and Long-Lived Assets [Line Items]
Net income (loss) including noncontrolling interest									(1,014,300)	518,400	242,900
Revenues from external customers									1,411,800	2,367,000	1,332,700
Interest and other income (loss)									(9,900)	(13,400)	0
Interest expense, net of capitalization									0	0	0
Income tax (benefit) expense									(244,200)	115,600	68,100
Impairment of assets									1,152,500
Depreciation, depletion and amortization									749,400	878,700	519,500
Accretion of asset retirement obligations									36,600	34,400	19,500
Amortization of undeveloped leases									17,200	23,100
Deferred income tax charge (benefit)									(244,200)	111,800
Additions to property, plant, equipment									623,100	2,193,300	1,343,500
Total assets at year-end	6,915,500				8,043,300				6,915,500	8,043,300	6,342,900
Exploration and Production | Operating Segments | UNITED STATES | Previously Reported
Revenues from External Customers and Long-Lived Assets [Line Items]
Amortization of undeveloped leases											36,800
Deferred income tax charge (benefit)											68,100
Exploration and Production | Operating Segments | Canada
Revenues from External Customers and Long-Lived Assets [Line Items]
Net income (loss) including noncontrolling interest									(35,000)	(4,300)	51,100
Revenues from external customers									345,800	447,000	470,500
Interest and other income (loss)									800	(1,500)	0
Interest expense, net of capitalization									(500)	(100)	0
Income tax (benefit) expense									(21,400)	(2,900)	14,500
Impairment of assets									0
Depreciation, depletion and amortization									213,200	243,000	232,400
Accretion of asset retirement obligations									5,500	6,100	7,600
Amortization of undeveloped leases									400	1,300
Deferred income tax charge (benefit)									(10,600)	14,000
Additions to property, plant, equipment									118,300	284,100	373,800
Total assets at year-end	2,404,100				2,303,700				2,404,100	2,303,700	1,711,900
Exploration and Production | Operating Segments | Canada | Previously Reported
Revenues from External Customers and Long-Lived Assets [Line Items]
Amortization of undeveloped leases											800
Deferred income tax charge (benefit)											16,500
Exploration and Production | Operating Segments | Other
Revenues from External Customers and Long-Lived Assets [Line Items]
Net income (loss) including noncontrolling interest									(85,600)	(53,500)	(16,600)
Revenues from external customers									1,800	11,600	22,200
Interest and other income (loss)									800	(900)	0
Interest expense, net of capitalization									(400)	(400)	200
Income tax (benefit) expense									2,100	(12,400)	(25,300)
Impairment of assets									39,700
Depreciation, depletion and amortization									2,300	3,500	3,500
Accretion of asset retirement obligations									0	0	0
Amortization of undeveloped leases									9,100	3,600
Deferred income tax charge (benefit)									1,900	(13,400)
Additions to property, plant, equipment									15,200	69,800	15,900
Total assets at year-end	$ 267,700				$ 308,600				$ 267,700	$ 308,600	188,100
Exploration and Production | Operating Segments | Other | Previously Reported
Revenues from External Customers and Long-Lived Assets [Line Items]
Amortization of undeveloped leases											2,500
Deferred income tax charge (benefit)											$ (25,700)

[1] Reclassified to conform with current presentation