Property, Plant and Equipment (Narrative) (Details) - USD ($)	1 Months Ended	3 Months Ended			6 Months Ended		12 Months Ended
	Jan. 31, 2017	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Apr. 01, 2017
Property Plant And Equipment [Line Items]
Total capitalized exploratory well costs		$ 206,371,000	$ 175,640,000	$ 175,534,000	$ 206,371,000	$ 175,534,000	$ 175,640,000	$ 148,500,000
Capitalized well costs charged to expense					0	8,360,000
Exploratory well costs capitalized more than one year		171,600,000			171,600,000
Proceeds from sales of property, plant and equipment					623,000	64,303,000
Gain (loss) on sale of assets		220,000		$ (1,333,000)	(118,000)	130,648,000
Athabasca Oil Corporation [Member]
Property Plant And Equipment [Line Items]
Cost of acquired assets, cash paid								206,700,000
Cost of acquired assets, carried interest								168,000,000
Carried interest paid		75,400,000			75,400,000
Canadian Subsidiary [Member] | Athabasca Oil Corporation [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						132,400,000	132,400,000
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
Montney Natural Gas Fields [Member]
Property Plant And Equipment [Line Items]
Proceeds from sales of property, plant and equipment								414,100,000
Deferred gain on sale of property		170,200,000			$ 170,200,000			$ 187,000,000
Deferred gain, period of recognition					18 years
Amortization of deferred gain					$ 3,800,000	$ 3,400,000
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		52,600,000			52,600,000
United States [Member]
Property Plant And Equipment [Line Items]
Exploratory well costs capitalized more than one year		27,800,000			27,800,000
Malaysia [Member]
Property Plant And Equipment [Line Items]
Exploratory well costs capitalized more than one year		$ 20,800,000			$ 20,800,000
Kaybob Duvernay Lands, Alberta [Member]
Property Plant And Equipment [Line Items]
Interest in assets acquired								70.00%
Placid Montney Lands, Alberta [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		6.37%			6.37%				6.67%
Unitization and Unit Operating Agreement [Member] | Kakap Field Block K Malaysia [Member]
Property Plant And Equipment [Line Items]
Redetermination expense								$ 39,100,000
Redetermination expense, net of tax								$ 24,100,000
Redetermination liability		$ 17,300,000			$ 17,300,000
Unitization Framework Agreement [Member] | Kakap Field Block K Malaysia [Member]
Property Plant And Equipment [Line Items]
Redetermination expense			15,000,000
Redetermination expense, net of tax			$ 9,300,000
Redetermination liability		$ 15,000,000			$ 15,000,000