Property, Plant and Equipment (Narrative) (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended				6 Months Ended
	Jan. 31, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Apr. 01, 2017	Dec. 31, 2015
Property Plant And Equipment [Line Items]
Total capitalized exploratory well costs		$ 175,534		$ 148,500	$ 128,109	$ 175,534	$ 128,109		$ 130,514
Exploratory well costs capitalized more than one year		117,600				117,600
Proceeds from sales of property, plant and equipment						64,303	1,153,325
Gain (loss) on sale of assets		(1,334)			3,809	130,648	3,831
Impairment of assets							95,088
Athabasca Oil Corporation [Member]
Property Plant And Equipment [Line Items]
Cost of acquired assets, carried interest					168,000
Carried interest paid		24,700				$ 24,700
Cost of acquired assets, interest carry period						5 years
Western Canada [Member]
Property Plant And Equipment [Line Items]
Proceeds from sales of property, plant and equipment	$ 49,000
Asset retirement obligation settled	85,900
Gain (loss) on sale of assets			$ 132,400
Syncrude [Member]
Property Plant And Equipment [Line Items]
Gain (loss) on sale of assets					$ 71,700
Percentage of interest in oil and gas property sold during period					5.00%
Eagle Ford Shale [Member]
Property Plant And Equipment [Line Items]
Proceeds from sales of property, plant and equipment	$ 14,800
Gain (loss) on sale of assets			$ 0
Montney Natural Gas Fields [Member]
Property Plant And Equipment [Line Items]
Deferred gain on sale of property		179,800			$ 187,000	$ 179,800	$ 187,000
Amortization of deferred gain on sale of property						$ 3,400
Deferred gain, period of recognition						19 years
Murphy Oil Corp [Member]
Property Plant And Equipment [Line Items]
Cost of acquired assets, cash paid					206,700
Murphy Oil Corp [Member] | Syncrude [Member]
Property Plant And Equipment [Line Items]
Proceeds from sales of property, plant and equipment					739,100
Canadian Subsidiary [Member] | Athabasca Oil Corporation [Member]
Property Plant And Equipment [Line Items]
Cost of acquired assets					$ 375,000
Brunei [Member]
Property Plant And Equipment [Line Items]
Exploratory well costs capitalized more than one year		70,400				$ 70,400
Vietnam [Member]
Property Plant And Equipment [Line Items]
Exploratory well costs capitalized more than one year		43,200				43,200
Malaysia [Member]
Property Plant And Equipment [Line Items]
Exploratory well costs capitalized more than one year		4,000				4,000
Redetermination expense				$ 39,100
Reduction in capital lease obligations		56,700				$ 56,700
Kaybob Duvernay Lands, Alberta [Member]
Property Plant And Equipment [Line Items]
Redetermination expense paid		$ 21,800
Interest in assets acquired					70.00%
Working interest			8.60%					6.70%
Montney Lands, Alberta [Member]
Property Plant And Equipment [Line Items]
Interest in assets acquired					30.00%