Supplemental Oil and Gas Information (Unaudited) - Results of Operations for Oil and Gas Producing Activities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenues
Revenues	$ 1,751,700	$ 2,817,100	$ 1,806,500
Other operating revenues	7,700	8,500	18,900
Total revenues	1,759,400	2,825,600	1,825,400
Costs and expenses
Lease operating expenses	600,100	605,200	353,800
Severance and ad valorem taxes	28,500	48,000	52,100
Transportation, gathering and processing	172,399	176,315	75,043
Restructuring expenses	49,994	0	0
Exploration costs charged to expense	59,700	67,300	61,600
Undeveloped lease amortization	26,800	28,000	40,100
Depreciation, depletion and amortization	964,900	1,125,200	755,400
Accretion of asset retirement obligations	42,200	40,500	27,200
Impairment of assets	1,206,284	0	20,000	[1]
Selling and general expenses	48,800	126,800	99,300
Other expenses (benefits)	21,000	47,400	6,200
Total costs and expenses	3,157,800	2,264,700	1,490,700
Results of operations before taxes	(1,398,400)	560,900	334,700
Income tax expense (benefit)	(263,500)	100,300	57,300
Results of operations	(1,134,900)	460,600	277,400
Net natural gas revenue
Costs and expenses
Restructuring expenses	1,200
Impairment of assets	1,192,200
Crude Oil and Natural Gas Liquids
Revenues
Revenues	1,511,600	2,584,800	1,586,600
Natural Gas
Revenues
Revenues	240,100	232,300	219,900
U.S.
Revenues
Revenues	1,405,300	2,359,700	1,331,300
Other operating revenues	6,500	7,300	1,400
Total revenues	1,411,800	2,367,000	1,332,700
Costs and expenses
Lease operating expenses	476,900	461,500	230,500
Severance and ad valorem taxes	27,200	46,600	50,900
Transportation, gathering and processing	127,700	140,800	43,100
Exploration costs charged to expense	35,500	21,400	29,400
Undeveloped lease amortization	17,200	23,100	36,800
Depreciation, depletion and amortization	749,400	878,700	519,500
Accretion of asset retirement obligations	36,600	34,400	19,500
Impairment of assets			20,000
Selling and general expenses	24,600	74,300	49,000
Other expenses (benefits)	21,500	52,200	23,000
Total costs and expenses	2,670,300	1,733,000	1,021,700
Results of operations before taxes	(1,258,500)	634,000	311,000
Income tax expense (benefit)	(244,200)	115,600	68,100
Results of operations	(1,014,300)	518,400	242,900
U.S. | Net natural gas revenue
Costs and expenses
Restructuring expenses	1,200
Impairment of assets	1,152,500
U.S. | Crude Oil and Natural Gas Liquids
Revenues
Revenues	1,335,800	2,285,800	1,277,700
U.S. | Natural Gas
Revenues
Revenues	69,400	73,900	53,600
Canada | Net natural gas revenue
Costs and expenses
Restructuring expenses	0
Canada | Conventional gas
Revenues
Revenues	344,600	445,800	469,100
Other operating revenues	1,200	1,200	1,400
Total revenues	345,800	447,000	470,500
Costs and expenses
Lease operating expenses	121,600	142,400	122,600
Severance and ad valorem taxes	1,300	1,400	1,200
Transportation, gathering and processing	44,700	35,500	31,900
Exploration costs charged to expense	600	600	600
Undeveloped lease amortization	400	1,300	800
Depreciation, depletion and amortization	213,200	243,000	232,400
Accretion of asset retirement obligations	5,600	6,100	7,700
Impairment of assets			0
Selling and general expenses	17,100	30,000	26,800
Other expenses (benefits)	(2,300)	(6,100)	(19,100)
Total costs and expenses	402,200	454,200	404,900
Results of operations before taxes	(56,400)	(7,200)	65,600
Income tax expense (benefit)	(21,400)	(2,900)	14,500
Results of operations	(35,000)	(4,300)	51,100
Canada | Conventional gas | Net natural gas revenue
Costs and expenses
Impairment of assets	0
Canada | Crude Oil and Natural Gas Liquids | Conventional gas
Revenues
Revenues	174,000	287,400	302,800
Canada | Natural Gas | Conventional gas
Revenues
Revenues	170,600	158,400	166,300
Other
Revenues
Revenues	1,800	11,600	6,100
Other operating revenues	0	0	16,100
Total revenues	1,800	11,600	22,200
Costs and expenses
Lease operating expenses	1,600	1,300	700
Severance and ad valorem taxes	0	0	0
Transportation, gathering and processing	0	0	0
Exploration costs charged to expense	23,600	45,300	31,600
Undeveloped lease amortization	9,200	3,600	2,500
Depreciation, depletion and amortization	2,300	3,500	3,500
Accretion of asset retirement obligations	0	0	0
Impairment of assets			0
Selling and general expenses	7,100	22,500	23,500
Other expenses (benefits)	1,800	1,300	2,300
Total costs and expenses	85,300	77,500	64,100
Results of operations before taxes	(83,500)	(65,900)	(41,900)
Income tax expense (benefit)	2,100	(12,400)	(25,300)
Results of operations	(85,600)	(53,500)	(16,600)
Other | Net natural gas revenue
Costs and expenses
Restructuring expenses	0
Impairment of assets	39,700
Other | Crude Oil and Natural Gas Liquids
Revenues
Revenues	1,800	11,600	6,100
Other | Natural Gas
Revenues
Revenues	$ 0	$ 0	$ 0

[1] Reclassified to conform with current presentation