Property, Plant and Equipment Narrative - (Details) $ in Thousands		9 Months Ended			12 Months Ended
	Jul. 31, 2019 USD ($) entity	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)		Dec. 31, 2018 USD ($)	Dec. 31, 2016 USD ($)	Jan. 01, 2019 USD ($)	Dec. 31, 2017 USD ($)
Property, Plant and Equipment [Line Items]
Well Costs		$ 280,735	$ 189,928		$ 207,855			$ 155,103
Exploratory well costs capitalized more than one year		216,000
Proceeds from sales of property, plant and equipment		19,072	921	[1]
Gain (loss) on sale of assets		363	6	[1]
Athabasca Oil Corporation | Canadian Subsidiary
Property, Plant and Equipment [Line Items]
Cost of acquired assets, carried interest						$ 168,000
Carried interest paid		131,600
Malaysia Subsidiaries
Property, Plant and Equipment [Line Items]
Number of businesses divested | entity	2
Cash received from divestiture	$ 2,000,000
Gain (loss) on disposition of business	960,000	960,000
Contingent bonus payment	$ 100,000
Montney Natural Gas Fields
Property, Plant and Equipment [Line Items]
Proceeds from sales of property, plant and equipment						$ 414,100
Gain (loss) on sale of assets			$ 5,700		187,000
Deferred gain on sale of property					$ 116,800		$ 116,800
Brunei
Property, Plant and Equipment [Line Items]
Exploratory well costs capitalized more than one year		57,400
Vietnam
Property, Plant and Equipment [Line Items]
Exploratory well costs capitalized more than one year		67,500
Gulf of Mexico
Property, Plant and Equipment [Line Items]
Exploratory well costs capitalized more than one year		63,600
United States
Property, Plant and Equipment [Line Items]
Exploratory well costs capitalized more than one year		$ 27,500
Kaybob Duvernay Lands, Alberta | Canadian Subsidiary
Property, Plant and Equipment [Line Items]
Interest in assets acquired						70.00%
Placid Montney Lands, Alberta | Canadian Subsidiary
Property, Plant and Equipment [Line Items]
Interest in assets acquired						30.00%

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