Property, Plant and Equipment (Narrative) (Details) - USD ($)	1 Months Ended	3 Months Ended	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017		Dec. 31, 2016		Apr. 01, 2017	Dec. 31, 2015
Property Plant And Equipment [Line Items]
Total capitalized exploratory well costs	$ 229,084,000	$ 175,640,000	$ 229,084,000	$ 175,640,000		$ 148,500,000			$ 130,514,000
Exploratory well costs capitalized more than one year	$ 167,900,000		167,900,000
Proceeds from sales of property, plant and equipment			1,383,000	69,506,000		1,155,144,000
Asset retirement obligation settled			23,398,000	29,111,000
Gain (loss) on sale of assets			54,000	127,434,000		1,663,000
Costs Incurred Acquisition Of Oil And Gas Properties			797,300,000	71,100,000		225,300,000
Cost of acquired assets			797,300,000	71,100,000		225,300,000
Impairment of assets			20,000,000	0		95,088,000	[1]
Redetermination expense			$ 11,332,000	15,000,000	[1]	39,100,000	[1]
MP Gulf Of Mexico LLC [Member]
Property Plant And Equipment [Line Items]
Parent ownership interest	80.00%		80.00%
MP Gulf Of Mexico LLC [Member] | Petrobas America Inc [Member]
Property Plant And Equipment [Line Items]
Noncontrolling ownership interest	20.00%		20.00%
Athabasca Oil Corporation [Member]
Property Plant And Equipment [Line Items]
Cost of acquired assets, cash paid						206,700,000
Athabasca Oil Corporation [Member] | Canadian Subsidiary [Member]
Property Plant And Equipment [Line Items]
Costs Incurred Acquisition Of Oil And Gas Properties						375,000,000
Cost of acquired assets						375,000,000
Petrobas America Inc [Member]
Property Plant And Equipment [Line Items]
Acquisition, cash paid	$ 794,623,000
Petrobas America Inc [Member] | Sales Thresholds 2019 to 2025 [Member]
Property Plant And Equipment [Line Items]
Contingent consideration, maximum	150,000,000		$ 150,000,000
Petrobas America Inc [Member] | Enhanced Oil Recovery Projects [Member]
Property Plant And Equipment [Line Items]
Contingent consideration, maximum	50,000,000		50,000,000
Western Canada [Member]
Property Plant And Equipment [Line Items]
Proceeds from sales of property, plant and equipment				48,800,000
Asset retirement obligation settled				85,900,000
Gain (loss) on sale of assets				129,000,000
Eagle Ford Shale [Member]
Property Plant And Equipment [Line Items]
Proceeds from sales of property, plant and equipment				19,600,000
Gain (loss) on sale of assets				0
Syncrude [Member]
Property Plant And Equipment [Line Items]
Proceeds from sales of property, plant and equipment						739,100,000
Gain (loss) on sale of assets						$ 71,700,000
Syncrude [Member] | Canadian Subsidiary [Member]
Property Plant And Equipment [Line Items]
Percentage of interest in oil and gas property sold during period						5.00%
Montney Natural Gas Fields [Member]
Property Plant And Equipment [Line Items]
Proceeds from sales of property, plant and equipment						$ 414,100,000
Gain (loss) on sale of assets			7,600,000	7,100,000
Deferred gain on sale of property	160,200,000		$ 160,200,000			187,000,000
Deferred gain, period of recognition			17 years
Brunei [Member]
Property Plant And Equipment [Line Items]
Exploratory well costs capitalized more than one year	55,800,000		$ 55,800,000
Vietnam [Member]
Property Plant And Equipment [Line Items]
Exploratory well costs capitalized more than one year	63,500,000		63,500,000
United States [Member]
Property Plant And Equipment [Line Items]
Costs Incurred Acquisition Of Oil And Gas Properties			797,100,000	58,100,000		18,600,000
Cost of acquired assets			797,100,000	$ 58,100,000		18,600,000
Malaysia [Member]
Property Plant And Equipment [Line Items]
Exploratory well costs capitalized more than one year	21,200,000		21,200,000
Kaybob Duvernay Lands, Alberta [Member] | Athabasca Oil Corporation [Member]
Property Plant And Equipment [Line Items]
Cost of acquired assets, carried interest						$ 168,000,000
Carried interest paid	102,000,000		102,000,000
Kaybob Duvernay Lands, Alberta [Member] | Athabasca Oil Corporation [Member] | Canadian Subsidiary [Member]
Property Plant And Equipment [Line Items]
Interest in assets acquired						70.00%
Placid Montney Lands, Alberta [Member] | Athabasca Oil Corporation [Member] | Canadian Subsidiary [Member]
Property Plant And Equipment [Line Items]
Interest in assets acquired						30.00%
Kakap Field Block K Malaysia [Member]
Property Plant And Equipment [Line Items]
Working interest		6.35%		6.35%				6.67%
Redetermination expense		$ 15,000,000				$ 39,100,000
Redetermination expense, net of tax		$ 9,300,000				$ 24,100,000
Redetermination liability	$ 17,300,000		$ 17,300,000

[1] Reclassified to conform to current presentation (see Note B).