Supplemental Oil and Gas Information - Results of Operations for Oil and Gas Producing Activities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018		Dec. 31, 2017
Revenues
Revenues	$ 2,817,100	$ 1,806,500		$ 1,300,400
Other operating revenues	8,500	18,900		129,400
Total revenues	2,825,600	1,825,400		1,429,800
Costs and expenses
Lease operating expenses	605,200	353,800		299,600
Severance and ad valorem taxes	48,000	52,100		43,700
Transportation, gathering and processing	176,315	75,043	[1]	0	[1]
Exploration costs charged to expense	67,300	61,600		58,600
Undeveloped lease amortization	28,000	40,100		61,800
Depreciation, depletion and amortization	1,125,200	755,400		735,300
Accretion of asset retirement obligations	40,500	27,200		25,300
Impairment of assets		20,000
Selling and general expenses	126,800	99,300		109,700
Results of Operations, Expense, Other				96,000
Other expenses (benefits)	47,400
Other expenses		6,200
Total costs and expenses	2,264,700	1,490,700		1,356,300
Results of operations before taxes	560,900	334,700		73,500
Income tax expense (benefit)	100,300	57,300		7,400
Results of operations	460,600	277,400		66,100
Crude Oil and Natural Gas Liquids
Revenues
Revenues	2,584,800	1,586,600		1,107,400
Natural Gas
Revenues
Revenues	232,300	219,900		193,000
United States
Revenues
Revenues	2,359,700	1,331,300		941,600
Other operating revenues	7,300	1,400		2,700
Total revenues	2,367,000	1,332,700		944,300
Costs and expenses
Lease operating expenses	461,500	230,500		198,500
Severance and ad valorem taxes	46,600	50,900		42,200
Transportation, gathering and processing	140,800	43,100
Exploration costs charged to expense	21,400	29,400		7,800
Undeveloped lease amortization	23,100	36,800		60,200
Depreciation, depletion and amortization	878,700	519,500		546,100
Accretion of asset retirement obligations	34,400	19,500		17,400
Impairment of assets		20,000
Selling and general expenses	74,300	49,000		61,800
Results of Operations, Expense, Other				20,000
Other expenses (benefits)	52,200
Other expenses		23,000
Total costs and expenses	1,733,000	1,021,700		954,000
Results of operations before taxes	634,000	311,000		(9,700)
Income tax expense (benefit)	115,600	68,100		(800)
Results of operations	518,400	242,900		(8,900)
United States | Crude Oil and Natural Gas Liquids
Revenues
Revenues	2,285,800	1,277,700		903,700
United States | Natural Gas
Revenues
Revenues	73,900	53,600		37,900
Canada | Canada
Revenues
Revenues	445,800	469,100		358,800
Other operating revenues	1,200	1,400		126,700
Total revenues	447,000	470,500		485,500
Costs and expenses
Lease operating expenses	142,400	122,600		101,100
Severance and ad valorem taxes	1,400	1,200		1,500
Transportation, gathering and processing	35,500	31,900
Exploration costs charged to expense	600	600		500
Undeveloped lease amortization	1,300	800		1,600
Depreciation, depletion and amortization	243,000	232,400		185,400
Accretion of asset retirement obligations	6,100	7,700		7,900
Impairment of assets		0
Selling and general expenses	30,000	26,800		28,300
Results of Operations, Expense, Other				2,300
Other expenses (benefits)	(6,100)
Other expenses		(19,100)
Total costs and expenses	454,200	404,900		328,600
Results of operations before taxes	(7,200)	65,600		156,900
Income tax expense (benefit)	(2,900)	14,500		44,400
Results of operations	(4,300)	51,100		112,500
Canada | Crude Oil and Natural Gas Liquids | Canada
Revenues
Revenues	287,400	302,800		203,700
Canada | Natural Gas | Canada
Revenues
Revenues	158,400	166,300		155,100
Other
Revenues
Revenues	11,600	6,100		0
Other operating revenues	0	16,100		0
Total revenues	11,600	22,200		0
Costs and expenses
Lease operating expenses	1,300	700		0
Severance and ad valorem taxes	0	0		0
Transportation, gathering and processing	0	0
Exploration costs charged to expense	45,300	31,600		50,300
Undeveloped lease amortization	3,600	2,500		0
Depreciation, depletion and amortization	3,500	3,500		3,800
Accretion of asset retirement obligations	0	0		0
Impairment of assets		0
Selling and general expenses	22,500	23,500		19,600
Results of Operations, Expense, Other				73,700
Other expenses (benefits)	1,300
Other expenses		2,300
Total costs and expenses	77,500	64,100		73,700
Results of operations before taxes	(65,900)	(41,900)		(73,700)
Income tax expense (benefit)	(12,400)	(25,300)		(36,200)
Results of operations	(53,500)	(16,600)		(37,500)
Other | Crude Oil and Natural Gas Liquids
Revenues
Revenues	11,600	6,100		0
Other | Natural Gas
Revenues
Revenues	$ 0	$ 0		$ 0

[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.