Property, Plant and Equipment (Narrative) (Details) - USD ($) $ in Thousands	1 Months Ended		3 Months Ended			9 Months Ended		12 Months Ended	24 Months Ended
	Sep. 30, 2016	Apr. 30, 2016	Sep. 30, 2016	Jun. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015	Dec. 31, 2015
Property Plant And Equipment [Line Items]
Exploratory well costs capitalized more than one year	$ 117,600		$ 117,600			$ 117,600
Proceeds from sales of property, plant and equipment						1,154,623	$ 423,842
Impairment of assets					$ 2,300,974	95,088	2,300,974
Syncrude Canada Ltd. [Member]
Property Plant And Equipment [Line Items]
Gain (loss) on sale of assets, net of taxes			71,700
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
Deferred gain on sale of property		$ 187,000
Amortization of deferred gain						$ 3,400
Deferred gain, period of recognition						20 years
Brunei [Member]
Property Plant And Equipment [Line Items]
Exploratory well costs capitalized more than one year	64,500		64,500			$ 64,500
Vietnam [Member]
Property Plant And Equipment [Line Items]
Exploratory well costs capitalized more than one year	31,400		31,400			31,400
Malaysia [Member]
Property Plant And Equipment [Line Items]
Exploratory well costs capitalized more than one year	21,700		$ 21,700			21,700
Proceeds from sales of property, plant and equipment								$ 417,200
Gain (loss) on sale of assets, net of taxes								$ 218,800
Impairment of assets							$ 1,472,600
Percentage of interest in oil and gas property sold during period								10.00%	30.00%
Kaybob Duvernay Lands, Alberta [Member]
Property Plant And Equipment [Line Items]
Interest in assets acquired				70.00%
Montney Lands, Alberta [Member]
Property Plant And Equipment [Line Items]
Interest in assets acquired				30.00%
Canada [Member]
Property Plant And Equipment [Line Items]
Impairment of assets						$ 95,088
Parent Company [Member] | Syncrude Canada Ltd. [Member]
Property Plant And Equipment [Line Items]
Proceeds from sales of property, plant and equipment	$ 739,100
Parent Company [Member] | Montney Lands, Alberta [Member]
Property Plant And Equipment [Line Items]
Cost of acquired assets, cash paid				$ 206,700
Athabasca Oil Corporation [Member]
Property Plant And Equipment [Line Items]
Cost of acquired assets, carried interest				$ 168,000
Cost of acquired assets, interest carry period						5 years
Athabasca Oil Corporation [Member] | Kaybob Duvernay Lands, Alberta [Member]
Property Plant And Equipment [Line Items]
Interest in assets acquired				70.00%
Athabasca Oil Corporation [Member] | Montney Lands, Alberta [Member]
Property Plant And Equipment [Line Items]
Interest in assets acquired				30.00%
Athabasca Oil Corporation [Member] | Subsidiary [Member]
Property Plant And Equipment [Line Items]
Cost of acquired assets				$ 375,000