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Supplemental Oil and Gas Information (Capitalized Costs Relating to Oil and Gas Producing Activities) (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Capitalized Costs Relating to Oil and Gas Producing Activities, by Geographic Area [Line Items]    
Unproved oil and gas properties $ 830.3 $ 849.0
Proved oil and gas properties 19,349.5 19,774.0
Gross capitalized costs 20,179.8 20,623.0
Accumulated depreciation, depletion and amortization    
Unproved oil and gas properties (402.0) (406.6)
Proved oil and gas properties (11,696.2) (12,049.7)
Net capitalized costs 8,081.6 8,166.7
United States [Member]    
Capitalized Costs Relating to Oil and Gas Producing Activities, by Geographic Area [Line Items]    
Unproved oil and gas properties 360.9 360.8
Proved oil and gas properties 9,606.4 9,384.6
Gross capitalized costs 9,967.3 9,745.4
Accumulated depreciation, depletion and amortization    
Unproved oil and gas properties (149.5) (151.2)
Proved oil and gas properties (4,893.8) (4,605.9)
Net capitalized costs 4,924.0 4,988.3
Canada [Member]    
Capitalized Costs Relating to Oil and Gas Producing Activities, by Geographic Area [Line Items]    
Unproved oil and gas properties 286.8 315.6
Proved oil and gas properties 3,603.4 4,241.6
Gross capitalized costs 3,890.2 4,557.2
Accumulated depreciation, depletion and amortization    
Unproved oil and gas properties (230.7) (233.6)
Proved oil and gas properties (2,027.9) (2,877.2)
Net capitalized costs 1,631.6 1,446.4
Malaysia [Member]    
Capitalized Costs Relating to Oil and Gas Producing Activities, by Geographic Area [Line Items]    
Unproved oil and gas properties 20.5 47.0
Proved oil and gas properties 6,139.7 6,147.8
Gross capitalized costs 6,160.2 6,194.8
Accumulated depreciation, depletion and amortization    
Proved oil and gas properties (4,774.5) (4,566.6)
Net capitalized costs 1,385.7 1,628.2
Other Regions [Member]    
Capitalized Costs Relating to Oil and Gas Producing Activities, by Geographic Area [Line Items]    
Unproved oil and gas properties 162.1 125.6
Gross capitalized costs 162.1 125.6
Accumulated depreciation, depletion and amortization    
Unproved oil and gas properties (21.8) (21.8)
Net capitalized costs $ 140.3 $ 103.8