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Debt - Additional Information (Detail)
 3 Months Ended
Mar. 31, 2016
USD ($)
Mar. 31, 2016
CAD
Mar. 31, 2015
USD ($)
Mar. 31, 2016
CAD
Dec. 31, 2015
USD ($)
Debt Instrument [Line Items]          
Debt maturing within twelve months $ 1,140,000,000        
Current debt obligations 524,000,000       $ 253,000,000
Tax-exempt bonds subject to re-pricing within next 12 months 381,000,000        
Debt maturing within twelve months classified as long-term 997,000,000        
Letters of credit outstanding under credit facility 965,000,000        
Loss on early extinguishment of debt (1,000,000)   $ (550,000,000)    
Tax-exempt Bonds [Member] | Variable Rate [Member]          
Debt Instrument [Line Items]          
Variable-rate tax-exempt bonds $ 491,000,000        
6.1% Senior Notes Due 2018 [Member]          
Debt Instrument [Line Items]          
Interest rate 6.10%     6.10%  
Net borrowings (repayments) of senior notes $ 10,000,000        
Debt instrument face amount 600,000,000        
Loss on early extinguishment of debt $ (1,000,000)        
Maturity period 2018 2018      
2.6% Senior Notes Mature in September 2016 [Member]          
Debt Instrument [Line Items]          
Senior note amount intended to refinance on a long-term basis $ 500,000,000        
Interest rate 2.60%     2.60%  
Senior Notes [Member]          
Debt Instrument [Line Items]          
Loss on early extinguishment of debt     (550,000,000)    
Early extinguishment of high-coupon senior notes     $ 2,000,000,000    
Canadian Credit Facility [Member]          
Debt Instrument [Line Items]          
Credit Facility, aggregate capacity | CAD       CAD 50,000,000  
2016 Canadian Credit Agreement [Member] | Intercompany Debt [Member]          
Debt Instrument [Line Items]          
Repayment of debt $ 280,000,000 CAD 370,000,000      
Cash [Member] | Tax-exempt Bonds [Member]          
Debt Instrument [Line Items]          
Repayment of tax exempt bonds 30,000,000        
Revolving Credit Facility [Member]          
Debt Instrument [Line Items]          
Credit Facility, aggregate capacity 2,250,000,000        
Outstanding borrowings under credit facility classified as long-term 116,000,000        
Outstanding borrowings under credit facility 458,000,000        
Letters of credit outstanding under credit facility $ 815,000,000        
Maturity date of credit facility Jul. 30, 2020 Jul. 30, 2020      
Credit Facility, remaining capacity $ 977,000,000        
Net borrowings (repayments) 438,000,000        
Letter of Credit Facilities [Member]          
Debt Instrument [Line Items]          
Credit Facility, aggregate capacity $ 2,400,000,000        
Maturity date of credit facility Dec. 31, 2018 Dec. 31, 2018      
Non Revolving Term Credit Facility [Member]          
Debt Instrument [Line Items]          
Credit Facility, aggregate capacity | CAD       460,000,000  
Canadian Term Loan [Member]          
Debt Instrument [Line Items]          
Credit Facility, aggregate capacity | CAD       500,000,000  
Credit Facility, remaining capacity | CAD       CAD 90,000,000  
Net borrowings (repayments) $ 20,000,000 CAD 27,000,000      
Canadian Term Loan [Member] | 2016 Canadian Credit Agreement [Member]          
Debt Instrument [Line Items]          
Net borrowings (repayments) $ 347,000,000 CAD 460,000,000