Supplemental Oil and Gas Information (Narrative) (Details) MMBbls in Millions		1 Months Ended	12 Months Ended
		Jan. 31, 2015	Dec. 31, 2018	Dec. 31, 2017 bbl	Dec. 31, 2017 MMcf	Dec. 31, 2017 ft³	Dec. 31, 2017 MMBbls	Dec. 31, 2017 $ / Mcf	Dec. 31, 2017	Dec. 31, 2016 ft³ MMBbls	Dec. 31, 2015 ft³ MMBbls	Dec. 31, 2014 ft³ MMBbls
Reserve Quantities [Line Items]
Proved reserves							328.1			329.0	456.2	441.8
Future net cash flows annual discount factor, percentage									10.00%
U.S. statutory rate				35.00%
Equity Method Investee [Member]
Reserve Quantities [Line Items]
Reserves attributable to investees accounted for my the equity method | bbl				0
Scenario, Plan [Member]
Reserve Quantities [Line Items]
U.S. statutory rate			21.00%
Oil Reserves [Member]
Reserve Quantities [Line Items]
Proved undeveloped reserve development window				5 years
Natural Gas Liquids Reserves [Member]
Reserve Quantities [Line Items]
Proved reserves							48.9			42.5	36.4	30.6
Proved undeveloped reserve development window				5 years
Natural Gas Reserves [Member]
Reserve Quantities [Line Items]
Proved reserves | ft³						1,927,100,000,000				1,878,000,000,000	1,688,800,000,000	1,704,700,000,000
Proved undeveloped reserve development window				5 years
Alberta, Canada [Member]
Reserve Quantities [Line Items]
Working interest											5.00%
Malaysia [Member] | Oil Reserves [Member]
Reserve Quantities [Line Items]
Proved reserves							52.2
Malaysia [Member] | Natural Gas Liquids Reserves [Member]
Reserve Quantities [Line Items]
Proved reserves							0.3			0.5	0.6	0.8
Malaysia [Member] | Natural Gas Reserves [Member]
Reserve Quantities [Line Items]
Proved reserves					491,300	491,400,000,000				539,700,000,000	546,800,000,000	635,600,000,000
Malaysia [Member] | Natural Gas Reserves [Member] | Held-for-sale [Member]
Reserve Quantities [Line Items]
Proved reserves | ft³						26,700
Expected sales proceeds | $ / Mcf								0.24
Canada [Member] | Natural Gas Liquids Reserves [Member]
Reserve Quantities [Line Items]
Proved reserves							5.6			5.6	0.4	0.7
Canada [Member] | Natural Gas Reserves [Member]
Reserve Quantities [Line Items]
Proved reserves | ft³						1,212,400,000,000				1,118,900,000,000	909,600,000,000	842,800,000,000
Canada [Member] | Synthetic Oil [Member]
Reserve Quantities [Line Items]
Proved reserves	[1]									0.0	114.8	105.6
Malaysia [Member]
Reserve Quantities [Line Items]
Percentage of interest in oil and gas property sold during period		10.00%

[1] All synthetic oil operations were sold in June 2016.