Property, Plant and Equipment (Narrative) (Details) - USD ($)		1 Months Ended	3 Months Ended	12 Months Ended
		Jan. 31, 2017	Dec. 31, 2017	Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015		Apr. 01, 2017	Dec. 31, 2014
Property Plant And Equipment [Line Items]
Proceeds from sales of property, plant and equipment				$ 69,506,000		$ 1,155,144,000		$ 423,911,000
Asset retirement obligation settled				29,111,000		20,589,000
Gain (loss) on sale of assets				127,434,000		1,663,000	[1]	154,155,000	[1]
Total capitalized exploratory well costs			$ 175,640,000	175,640,000		148,500,000		130,514,000			$ 120,455,000
Cost of acquired assets				71,100,000		225,300,000		12,600,000
Impairment of assets				0		95,088,000	[1]	2,493,156,000	[1]
Redetermination expense				15,000,000	[2]	39,100,000	[1]
Exploratory well costs capitalized more than one year			134,100,000	134,100,000
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]
Cost of acquired assets						375,000,000
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				7,100,000		5,100,000
Deferred gain on sale of property			181,700,000	$ 181,700,000		187,000,000
Deferred gain, period of recognition				18 years
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			$ 44,800,000	$ 44,800,000
Cost of acquired assets, interest carry period				5 years
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
Malaysia [Member]
Property Plant And Equipment [Line Items]
Impairment of assets	[2]							$ 1,480,600,000
Redetermination expense	[2]			$ 15,000,000		$ 39,100,000
Exploratory well costs capitalized more than one year			20,500,000	20,500,000
Brunei [Member]
Property Plant And Equipment [Line Items]
Exploratory well costs capitalized more than one year			70,400,000	70,400,000
Vietnam [Member]
Property Plant And Equipment [Line Items]
Exploratory well costs capitalized more than one year			$ 43,200,000	$ 43,200,000

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

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