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Debt - Credit Facility (Narrative) (Detail)
$ in Billions
Dec. 31, 2016
CAD
Dec. 31, 2016
USD ($)
Jun. 28, 2016
USD ($)
Dec. 31, 2015
CAD
Dec. 31, 2015
USD ($)
Revolving Credit Facility [Member]          
Debt Instrument [Line Items]          
Credit total available amount | $   $ 2     $ 2
Five Year Portion [Member]          
Debt Instrument [Line Items]          
Credit total available amount | $   1 $ 1    
One Year Plus One Year Term Out Portion [Member]          
Debt Instrument [Line Items]          
Credit total available amount | $   1 $ 1    
Letter of Credit [Member]          
Debt Instrument [Line Items]          
Letters of credit drawn | CAD CAD 320,000,000     CAD 375,000,000  
Maximum capacity under credit facility | CAD 600,000,000     600,000,000  
Restricted cash and cash equivalents | CAD 0     0  
Commercial Paper [Member]          
Debt Instrument [Line Items]          
Credit total available amount | $   $ 1      
Long-term Debt, Current Maturities | CAD CAD 0     CAD 0