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Significant Accounting Policies (Narrative) (Details) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Significant Accounting Policies [Line Items]      
Maturity of Canadian government securities 90 days 90 days 90 days
Canadian government securities with maturities greater than 90 days at the date of acquisition   $ 111,542,000  
Impairment of assets $ 0 95,088,000 [1] $ 2,493,156,000 [1]
Withholding tax on amounts repatriated 5.00%    
Stock Options [Member]      
Significant Accounting Policies [Line Items]      
Stock-based compensation, fair value assumption model Black-Scholes option pricing model    
Performance Based Restricted Stock [Member]      
Significant Accounting Policies [Line Items]      
Stock-based compensation, fair value assumption model Monte Carlo valuation model    
Stock-based compensation, vesting period 3 years    
Minimum [Member]      
Significant Accounting Policies [Line Items]      
Equity method investment, ownership percentage 20.00%    
Minimum [Member] | Stock Options [Member]      
Significant Accounting Policies [Line Items]      
Stock-based compensation, vesting period 2 years    
Maximum [Member]      
Significant Accounting Policies [Line Items]      
Equity method investment, ownership percentage 50.00%    
Maximum [Member] | Stock Options [Member]      
Significant Accounting Policies [Line Items]      
Stock-based compensation, vesting period 3 years    
Syncrude [Member] | Minimum [Member]      
Significant Accounting Policies [Line Items]      
Turnarounds for major processing units 2 years    
Syncrude [Member] | Maximum [Member]      
Significant Accounting Policies [Line Items]      
Turnarounds for major processing units 3 years    
Natural Gas Processing Facilities [Member] | Minimum [Member]      
Significant Accounting Policies [Line Items]      
Depreciable lives, straight-line method 20 years    
Natural Gas Processing Facilities [Member] | Maximum [Member]      
Significant Accounting Policies [Line Items]      
Depreciable lives, straight-line method 25 years    
Canada [Member]      
Significant Accounting Policies [Line Items]      
Impairment of assets   $ 95,088,000 $ 683,574,000
[1] Reclassified to conform to current presentation (see Note A).