Property, Plant and Equipment (Narrative) (Details) - USD ($) $ in Thousands		1 Months Ended	3 Months Ended				12 Months Ended
		Dec. 31, 2016	Mar. 31, 2019	Mar. 31, 2018		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2018	Apr. 01, 2017
Property Plant And Equipment [Line Items]
Total capitalized exploratory well costs			$ 227,126	$ 155,652		$ 155,103		$ 207,855
Capitalized well costs charged to expense			13,145	0
Exploratory well costs capitalized more than one year			149,100
Proceeds from sales of property, plant and equipment	[1]			260
Gain (loss) on sale of assets			12	(339)	[1]
MP Gulf Of Mexico LLC [Member] | Petrobas America Inc [Member]
Property Plant And Equipment [Line Items]
Noncontrolling ownership interest								20.00%
Athabasca Oil Corporation [Member] | Canadian Subsidiary [Member]
Property Plant And Equipment [Line Items]
Cost of acquired assets, carried interest							$ 168,000
Carried interest paid			124,000
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			116,000
Gain (loss) on sale of assets		$ 187,000		$ 1,900
Brunei [Member]
Property Plant And Equipment [Line Items]
Exploratory well costs capitalized more than one year			57,000
Vietnam [Member]
Property Plant And Equipment [Line Items]
Exploratory well costs capitalized more than one year			64,900
United States [Member]
Property Plant And Equipment [Line Items]
Exploratory well costs capitalized more than one year			$ 27,300
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%						6.67%
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
Unitization Framework Agreement [Member] | Kakap Field Block K Malaysia [Member]
Property Plant And Equipment [Line Items]
Redetermination expense						$ 15,000

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