Property, Plant and Equipment (Narrative) (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended			9 Months Ended		12 Months Ended
	Jan. 31, 2017	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Apr. 01, 2017
Property Plant And Equipment [Line Items]
Total capitalized exploratory well costs		$ 210,845	$ 175,640	$ 178,384	$ 210,845	$ 178,384	$ 175,640	$ 148,500
Exploratory well costs capitalized more than one year		164,000			164,000
Proceeds from sales of property, plant and equipment					1,128	69,146
Gain (loss) on sale of assets		124		$ 117	6	130,765
Redetermination expense		11,332			11,332
Canadian Subsidiary [Member] | Athabasca Oil Corporation [Member]
Property Plant And Equipment [Line Items]
Cost of acquired assets								375,000
Cost of acquired assets, cash paid								206,700
Cost of acquired assets, carried interest								168,000
Carried interest paid		93,100			93,100
Western Canada [Member]
Property Plant And Equipment [Line Items]
Proceeds from sales of property, plant and equipment	$ 48,800
Asset retirement obligation settled	$ 85,900
Gain (loss) on sale of assets						132,400	132,400
Eagle Ford Shale [Member]
Property Plant And Equipment [Line Items]
Proceeds from sales of property, plant and equipment							19,600
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
Deferred gain on sale of property		171,000			$ 171,000			$ 187,000
Deferred gain, period of recognition					18 years
Amortization of deferred gain					$ 5,700	$ 5,300
Brunei [Member]
Property Plant And Equipment [Line Items]
Exploratory well costs capitalized more than one year		55,900			55,900
Vietnam [Member]
Property Plant And Equipment [Line Items]
Exploratory well costs capitalized more than one year		59,800			59,800
United States [Member]
Property Plant And Equipment [Line Items]
Exploratory well costs capitalized more than one year		27,400			27,400
Malaysia [Member]
Property Plant And Equipment [Line Items]
Exploratory well costs capitalized more than one year		$ 20,900			$ 20,900
Kaybob Duvernay Lands, Alberta [Member] | Canadian Subsidiary [Member]
Property Plant And Equipment [Line Items]
Interest in assets acquired								70.00%
Placid Montney Lands, Alberta [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		6.37%			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
Redetermination expense, net of tax								$ 24,100
Redetermination liability		$ 17,300			$ 17,300
Unitization Framework Agreement [Member] | Kakap Field Block K Malaysia [Member]
Property Plant And Equipment [Line Items]
Redetermination expense			$ 15,000