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Compensation Plans (Narrative) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Liability For Cash-Settled Share-Based Payment Transactions $ 327 $ 208 $ 51
Liability for share-based payment recognized in accounts payable and accrued liabilities 152 88  
Liability for share-based payment recognized in other liabilities and provisions 175 120  
Additional Paid in Capital $ 1,358 1,358  
TSARs and SARs [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Years After Grant Date Rights Expire 5 years    
TSARs and SARs [Member] | Share-based Compensation Award, Tranche One [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Vesting Rights Percentage 30.00%    
Years After Rights Granted First Portion Of Rights Vest 1 year    
TSARs and SARs [Member] | Share-based Compensation Award, Tranche Two [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Vesting Rights Percentage 30.00%    
Years After Grant Date Additional Thirty Percent Granted Vest 2 years    
TSARs and SARs [Member] | Share-based Compensation Award, Tranche Three [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Years After Grant Date Rights Fully Vest 3 years    
TSARs and SARs, Granted 2015 [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Years After Grant Date Rights Expire 7 years    
Tandem Stock Appreciation Rights (TSARs) [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized $ 8 17  
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition 1 year 10 months 24 days    
Total Compensation Costs $ 12 39 (12)
PSUs and RSUs [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Years After Grant Date Rights Fully Vest 3 years    
Performance Tandem Stock Appreciation Rights [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Liability For Cash-Settled Share-Based Payment Transactions $ 0    
Total Compensation Costs $ (2) 2 (1)
Period over which performance targets are acheived 4 years    
Stock Appreciation Rights (SARs) [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized $ 4 7  
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition 1 year 6 months    
Total Compensation Costs $ 6 13 (5)
Performance Share Units [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Years After Grant Date Rights Fully Vest 3 years    
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized $ 53 60  
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition 1 year 1 month 6 days    
Total Compensation Costs $ 48 29 1
Share Based Compensation Arrangement By Share Based Payment Award Performance Measure Period 3 years    
Maximum Multiplication Of Original Performance Stock Units Granted May Be Awarded 2    
Deferred Share Units [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
DSU Expiration Date December 15    
Total Compensation Costs $ 3 7 (5)
Restricted Share Units [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Years After Grant Date Rights Fully Vest 3 years    
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized $ 99 117  
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition 1 year 1 month 6 days    
Total Compensation Costs $ 98 $ 84 $ (7)
Maximum [Member] | Deferred Share Units [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Conversion Percentage Of High Performance Results Awards Into Deferred Share Units 50.00%    
Minimum [Member] | Deferred Share Units [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Conversion Percentage Of High Performance Results Awards Into Deferred Share Units 25.00%