Supplemental Oil and Gas Information (Results of Operations for Oil andGas Producing Activities) (Details) - USD ($)		12 Months Ended
		Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Revenues
Revenues	[1]	$ 2,088,100,000		$ 1,799,500,000		$ 2,775,000,000
Other operating revenues	[1]	132,400,000		5,100,000		159,700,000
Total revenues	[1]	2,220,500,000		1,804,600,000		2,934,700,000
Costs and expenses
Lease operating expenses	[1]	468,400,000		559,400,000		832,300,000
Severance and ad valorem taxes	[1]	43,700,000		43,800,000		65,800,000
Exploration costs charged to expense	[1]	61,000,000		58,500,000		395,500,000
Undeveloped lease amortization	[1]	61,800,000		43,400,000		75,400,000
Depreciation, depletion and amortization	[1]	939,900,000		1,037,300,000		1,607,900,000
Accretion of asset retirement obligations	[1]	42,600,000		46,700,000		48,700,000
Impairment of assets		0		95,088,000	[2]	2,493,156,000	[2]
Redetermination expense		15,000,000	[1]	39,100,000	[2]
Deepwater rig contract exit costs						282,000,000
Selling and general expenses	[1]	123,700,000		147,400,000		177,200,000
Other expenses (benefits)	[1]	30,700,000		13,900,000		360,600,000
Total costs and expenses	[1]	1,786,800,000		2,084,600,000		6,056,600,000
Results of operations before taxes	[1]	433,700,000		(280,000,000)		(3,121,900,000)
Income tax expense (benefit)	[1]	137,100,000		(155,100,000)		(1,111,000,000)
Results of operations	[1]	296,600,000		(124,900,000)		(2,010,900,000)
Crude Oil and Natural Gas Liquids [Member]
Revenues
Revenues	[1]	1,756,900,000		1,506,800,000		2,351,500,000
Natural Gas [Member]
Revenues
Revenues	[1]	331,200,000		292,700,000		423,500,000
United States [Member]
Revenues
Revenues	[1]	951,200,000		685,800,000		1,247,300,000
Other operating revenues	[1]	2,700,000		(100,000)		6,300,000
Total revenues	[1]	953,900,000		685,700,000		1,253,600,000
Costs and expenses
Lease operating expenses	[1]	198,500,000		218,600,000		312,000,000
Severance and ad valorem taxes	[1]	42,200,000		37,000,000		55,900,000
Exploration costs charged to expense	[1]	7,800,000		6,100,000		258,200,000
Undeveloped lease amortization	[1]	60,200,000		38,400,000		59,200,000
Depreciation, depletion and amortization	[1]	546,100,000		600,500,000		794,900,000
Accretion of asset retirement obligations	[1]	17,400,000		17,100,000		20,200,000
Impairment of assets	[1]					329,000,000
Selling and general expenses	[1]	61,800,000		68,800,000		88,200,000
Other expenses (benefits)	[1]	20,000,000		(7,500,000)		288,700,000
Total costs and expenses	[1]	954,000,000		979,000,000		2,206,300,000
Results of operations before taxes	[1]	(100,000)		(293,300,000)		(952,700,000)
Income tax expense (benefit)	[1]	2,500,000		(87,900,000)		(337,000,000)
Results of operations	[1]	(2,600,000)		(205,400,000)		(615,700,000)
United States [Member] | Crude Oil and Natural Gas Liquids [Member]
Revenues
Revenues	[1]	913,300,000		650,700,000		1,176,900,000
United States [Member] | Natural Gas [Member]
Revenues
Revenues	[1]	37,900,000		35,100,000		70,400,000
Canada [Member]
Costs and expenses
Impairment of assets				95,088,000		683,574,000
Canada [Member] | Conventional [Member]
Revenues
Revenues	[1]	358,800,000		301,700,000		348,700,000
Other operating revenues	[1]	126,700,000		(700,000)		(2,400,000)
Total revenues	[1]	485,500,000		301,000,000		346,300,000
Costs and expenses
Lease operating expenses	[1]	101,100,000		102,600,000		102,400,000
Severance and ad valorem taxes	[1]	1,500,000		4,300,000		4,800,000
Exploration costs charged to expense	[1]	500,000		3,600,000		700,000
Undeveloped lease amortization	[1]	1,600,000		4,500,000		14,400,000
Depreciation, depletion and amortization	[1]	185,400,000		186,700,000		211,200,000
Accretion of asset retirement obligations	[1]	7,900,000		10,900,000		7,200,000
Impairment of assets	[1]			95,100,000		683,600,000
Selling and general expenses	[1]	28,300,000		28,600,000		25,500,000
Other expenses (benefits)	[1]	2,300,000		7,500,000		43,900,000
Total costs and expenses	[1]	328,600,000		443,800,000		1,093,700,000
Results of operations before taxes	[1]	156,900,000		(142,800,000)		(747,400,000)
Income tax expense (benefit)	[1]	44,400,000		(58,900,000)		(191,200,000)
Results of operations	[1]	112,500,000		(83,900,000)		(556,200,000)
Canada [Member] | Synthetic Oil [Member]
Revenues
Revenues	[1]			60,700,000		203,000,000
Other operating revenues	[1]			3,600,000		400,000
Total revenues	[1]			64,300,000		203,400,000
Costs and expenses
Lease operating expenses	[1]			69,800,000		166,000,000
Severance and ad valorem taxes	[1]			2,500,000		5,100,000
Depreciation, depletion and amortization	[1]			16,500,000		50,700,000
Accretion of asset retirement obligations	[1]			2,400,000		5,400,000
Selling and general expenses	[1]			500,000		1,000,000
Total costs and expenses	[1]			91,700,000		228,200,000
Results of operations before taxes	[1]			(27,400,000)		(24,800,000)
Income tax expense (benefit)	[1]			(75,400,000)		2,400,000
Results of operations	[1]			48,000,000		(27,200,000)
Canada [Member] | Crude Oil and Natural Gas Liquids [Member] | Conventional [Member]
Revenues
Revenues	[1]	203,700,000		171,700,000		181,000,000
Canada [Member] | Crude Oil and Natural Gas Liquids [Member] | Synthetic Oil [Member]
Revenues
Revenues	[1]			60,700,000		203,000,000
Canada [Member] | Natural Gas [Member] | Conventional [Member]
Revenues
Revenues	[1]	155,100,000		130,000,000		167,700,000
Malaysia [Member]
Revenues
Revenues	[1]	778,100,000		751,300,000		976,000,000
Other operating revenues	[1]	3,000,000		2,100,000		155,400,000
Total revenues	[1]	781,100,000		753,400,000		1,131,400,000
Costs and expenses
Lease operating expenses	[1]	168,800,000		168,400,000		251,900,000
Exploration costs charged to expense	[1]	2,400,000		5,200,000		37,600,000
Depreciation, depletion and amortization	[1]	204,600,000		227,700,000		544,900,000
Accretion of asset retirement obligations	[1]	17,300,000		16,300,000		15,900,000
Impairment of assets	[1]					1,480,600,000
Redetermination expense	[1]	15,000,000		39,100,000
Selling and general expenses	[1]	14,000,000		15,900,000		5,700,000
Other expenses (benefits)	[1]	8,400,000		23,800,000		15,900,000
Total costs and expenses	[1]	430,500,000		496,400,000		2,352,500,000
Results of operations before taxes	[1]	350,600,000		257,000,000		(1,221,100,000)
Income tax expense (benefit)	[1]	126,400,000		85,900,000		(567,900,000)
Results of operations	[1]	224,200,000		171,100,000		(653,200,000)
Malaysia [Member] | Crude Oil and Natural Gas Liquids [Member]
Revenues
Revenues	[1]	639,900,000		623,700,000		790,600,000
Malaysia [Member] | Natural Gas [Member]
Revenues
Revenues	[1]	138,200,000		127,600,000		185,400,000
Other Regions [Member]
Revenues
Other operating revenues	[1]			200,000
Total revenues	[1]			200,000
Costs and expenses
Exploration costs charged to expense	[1]	50,300,000		43,600,000		99,000,000
Undeveloped lease amortization	[1]			500,000		1,800,000
Depreciation, depletion and amortization	[1]	3,800,000		5,900,000		6,200,000
Selling and general expenses	[1]	19,600,000		33,600,000		56,800,000
Other expenses (benefits)	[1]			(9,900,000)		12,100,000
Total costs and expenses	[1]	73,700,000		73,700,000		175,900,000
Results of operations before taxes	[1]	(73,700,000)		(73,500,000)		(175,900,000)
Income tax expense (benefit)	[1]	(36,200,000)		(18,800,000)		(17,300,000)
Results of operations	[1]	$ (37,500,000)		$ (54,700,000)		$ (158,600,000)

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

[2] Reclassified to conform to current presentation (see Note A).