Supplemental Oil and Gas Information (Results of Operations for Oil andGas Producing Activities) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2018		Dec. 31, 2017	[2]	Dec. 31, 2016
Revenues
Revenues	[1]	$ 2,586.6	[2]	$ 2,078.5		$ 1,862.9	[3]
Other operating revenues	[1]	18.0	[2]	132.4		5.1	[3]
Total revenues	[1]	2,604.6	[2]	2,210.9		1,868.0	[3]
Costs and expenses
Lease operating expenses	[1]	555.9	[2]	468.4		559.4	[3]
Severance and ad valorem taxes	[1]	52.1	[2]	43.7		43.8	[3]
Exploration costs charged to expense	[1]	63.8	[2]	61.0		58.5	[3]
Undeveloped lease amortization	[1]	40.1	[2]	61.8		43.4	[3]
Depreciation, depletion and amortization	[1]	954.0	[2]	939.9		1,037.3	[3]
Accretion of asset retirement obligations	[1]	44.6	[2]	42.6		46.7	[3]
Impairment of assets		20.0	[2]			95.1	[3]
Redetermination expense		11.3	[1],[2]	15.0		39.1	[1],[3]
Selling and general expenses	[1]	110.1	[2]	123.7		131.7	[3]
Other expenses (benefits)	[1]	5.2	[2]	30.7		29.6	[3]
Total costs and expenses	[1]	1,857.1	[2]	1,786.8		2,084.6	[3]
Results of operations before taxes	[1]	747.5	[2]	424.1		(216.6)	[3]
Income tax expense (benefit)	[1]	200.6	[2]	133.8		(132.9)	[3]
Results of operations		546.9	[1],[2]	290.3	[1]	(83.7)	[3]
Crude Oil and Natural Gas Liquids [Member]
Revenues
Revenues	[1]	2,251.4	[2]	1,747.3		1,570.2	[3]
Natural Gas [Member]
Revenues
Revenues	[1]	335.2	[2]	331.2		292.7	[3]
United States [Member]
Revenues
Revenues		1,288.2	[2]	941.6		749.2	[3]
Other operating revenues		1.4	[2]	2.7		(0.1)	[3]
Total revenues		1,289.6	[2]	944.3		749.1	[3]
Costs and expenses
Lease operating expenses		230.5	[2]	198.5		218.6	[3]
Severance and ad valorem taxes		50.9	[2]	42.2		37.0	[3]
Exploration costs charged to expense		29.4	[2]	7.8		6.1	[3]
Undeveloped lease amortization		36.8	[2]	60.2		38.4	[3]
Depreciation, depletion and amortization		519.5	[2]	546.1		600.5	[3]
Accretion of asset retirement obligations		19.5	[2]	17.4		17.1	[3]
Impairment of assets	[2]	20.0
Selling and general expenses		49.0	[2]	61.8		54.0	[3]
Other expenses (benefits)		23.0	[2]	20.0		7.3	[3]
Total costs and expenses		978.6	[2]	954.0		979.0	[3]
Results of operations before taxes		311.0	[2]	(9.7)		(229.9)	[3]
Income tax expense (benefit)		68.1	[2]	(0.8)		(65.7)	[3]
Results of operations		242.9	[2]	(8.9)		(164.2)	[3]
United States [Member] | Crude Oil and Natural Gas Liquids [Member]
Revenues
Revenues		1,245.3	[2]	903.7		714.1	[3]
United States [Member] | Natural Gas [Member]
Revenues
Revenues		42.9	[2]	37.9		35.1	[3]
Canada [Member] | Conventional [Member]
Revenues
Revenues		438.8	[2]	358.8		301.7	[3]
Other operating revenues		(0.2)	[2]	126.7		(0.7)	[3]
Total revenues		438.6	[2]	485.5		301.0	[3]
Costs and expenses
Lease operating expenses		122.6	[2]	101.1		102.6	[3]
Severance and ad valorem taxes		1.2	[2]	1.5		4.3	[3]
Exploration costs charged to expense		0.6	[2]	0.5		3.6	[3]
Undeveloped lease amortization		0.8	[2]	1.6		4.5	[3]
Depreciation, depletion and amortization		232.4	[2]	185.4		186.7	[3]
Accretion of asset retirement obligations		7.7	[2]	7.9		10.9	[3]
Impairment of assets	[3]					95.1
Selling and general expenses		26.8	[2]	28.3		28.2	[3]
Other expenses (benefits)		(19.1)	[2]	2.3		7.9	[3]
Total costs and expenses		373.0	[2]	328.6		443.8	[3]
Results of operations before taxes		65.6	[2]	156.9		(142.8)	[3]
Income tax expense (benefit)		14.5	[2]	44.4		(58.9)	[3]
Results of operations		51.1	[2]	112.5		(83.9)	[3]
Canada [Member] | Synthetic Oil [Member]
Revenues
Revenues	[3]					60.7
Other operating revenues	[3]					3.6
Total revenues	[3]					64.3
Costs and expenses
Lease operating expenses	[3]					69.8
Severance and ad valorem taxes	[3]					2.5
Depreciation, depletion and amortization	[3]					16.5
Accretion of asset retirement obligations	[3]					2.4
Selling and general expenses	[3]					0.5
Total costs and expenses	[3]					91.7
Results of operations before taxes	[3]					(27.4)
Income tax expense (benefit)	[3]					(75.4)
Results of operations	[3]					48.0
Canada [Member] | Crude Oil and Natural Gas Liquids [Member] | Conventional [Member]
Revenues
Revenues		291.2	[2]	203.7		171.7	[3]
Canada [Member] | Crude Oil and Natural Gas Liquids [Member] | Synthetic Oil [Member]
Revenues
Revenues	[3]					60.7
Canada [Member] | Natural Gas [Member] | Conventional [Member]
Revenues
Revenues		147.6	[2]	155.1		130.0	[3]
Malaysia [Member]
Revenues
Revenues		853.5	[2]	778.1		751.3	[3]
Other operating revenues		0.7	[2]	3.0		2.1	[3]
Total revenues		854.2	[2]	781.1		753.4	[3]
Costs and expenses
Lease operating expenses		202.1	[2]	168.8		168.4	[3]
Exploration costs charged to expense		2.2	[2]	2.4		5.2	[3]
Depreciation, depletion and amortization		198.6	[2]	204.6		227.7	[3]
Accretion of asset retirement obligations		17.4	[2]	17.3		16.3	[3]
Redetermination expense		11.3	[2]	15.0		39.1	[3]
Selling and general expenses		10.8	[2]	14.0		15.4	[3]
Other expenses (benefits)		(1.0)	[2]	8.4		24.3	[3]
Total costs and expenses		441.4	[2]	430.5		496.4	[3]
Results of operations before taxes		412.8	[2]	350.6		257.0	[3]
Income tax expense (benefit)		143.3	[2]	126.4		85.9	[3]
Results of operations		269.5	[2]	224.2		171.1	[3]
Malaysia [Member] | Crude Oil and Natural Gas Liquids [Member]
Revenues
Revenues		708.8	[2]	639.9		623.7	[3]
Malaysia [Member] | Natural Gas [Member]
Revenues
Revenues		144.7	[2]	138.2		127.6	[3]
Other Regions [Member]
Revenues
Revenues	[2]	6.1
Other operating revenues		16.1	[2]			0.2	[3]
Total revenues		22.2	[2]			0.2	[3]
Costs and expenses
Lease operating expenses	[2]	0.7
Exploration costs charged to expense		31.6	[2]	50.3		43.6	[3]
Undeveloped lease amortization		2.5	[2]			0.5	[3]
Depreciation, depletion and amortization		3.5	[2]	3.8		5.9	[3]
Selling and general expenses		23.5	[2]	19.6		33.6	[3]
Other expenses (benefits)		2.3	[2]			(9.9)	[3]
Total costs and expenses		64.1	[2]	73.7		73.7	[3]
Results of operations before taxes		(41.9)	[2]	(73.7)		(73.5)	[3]
Income tax expense (benefit)		(25.3)	[2]	(36.2)		(18.8)	[3]
Results of operations		(16.6)	[2]	$ (37.5)		$ (54.7)	[3]
Other Regions [Member] | Crude Oil and Natural Gas Liquids [Member]
Revenues
Revenues	[2]	$ 6.1

[1] 1 Results exclude corporate overhead, interest and discontinued operations. 2018 includes noncontrolling interest in MP GOM.

[2] Results exclude corporate overhead, interest and discontinued operations. 2018 includes noncontrolling interest in MP GOM.

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