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Debt - Additional Information (Detail)
 1 Months Ended 12 Months Ended 12 Months Ended 1 Months Ended 12 Months Ended
Sep. 30, 2012
USD ($)
Dec. 31, 2012
USD ($)
Dec. 31, 2012
CAD
Dec. 31, 2011
USD ($)
Dec. 31, 2012
Revolving credit facility [Member]
USD ($)
Dec. 31, 2012
Letter of credit facilities [Member]
Nov. 30, 2012
Canadian credit facility [Member]
USD ($)
Dec. 31, 2012
Canadian credit facility [Member]
USD ($)
Dec. 31, 2012
Canadian credit facility [Member]
CAD
Debt Instrument [Line Items]                  
Debt maturing within twelve months   $ 688,000,000              
Outstanding borrowing under revolving credit facility   400,000,000   150,000,000         75,000,000
Canadian credit facility   75,000,000   137,000,000          
Tax-exempt bonds, current   161,000,000              
Tax-exempt bonds subject to re-pricing within next 12 months   475,000,000              
Debt maturing within twelve months classified as long-term   420,000,000              
Short Term Borrowings   743,000,000              
Variable-rate tax-exempt bonds   587,000,000              
Revolving credit facility         2,000,000,000        
Letters of credit outstanding revolving credit facility   933,000,000              
Unused and available credit capacity, revolving credit facility   667,000,000              
End period of maturity for debt instruments         May 09, 2016 Jun. 30, 2015   Nov. 07, 2017  
Letter of credit facilities, total capacity   505,000,000              
Unused and available credit capacity, letter of credit   13,000,000              
Start period of maturity for debt instruments           Jun. 30, 2013      
Canadian credit facility credit capacity     340,000,000           150,000,000
New Canadian credit facility             75,000,000    
Borrowings under revolving credit facility         250,000,000        
Net Repayment of Canadian credit facility debt               68,000,000  
Senior notes issued 500,000,000                
Interest rate of senior notes 2.90% 6.375%              
Senior notes maturity date Sep. 15, 2022                
Net proceeds from the debt issuance 495,000,000                
Portion of the proceeds used to repay   400,000,000              
Issuance of tax exemption bonds   43,000,000              
Repayment of tax exemption bonds   129,000,000              
Debt and capital lease payment in 2013   695,000,000              
Debt and capital lease payment in 2014   468,000,000              
Debt and capital lease payment in 2015   462,000,000              
Debt and capital lease payment in 2016   728,000,000              
Debt and capital lease payment in 2017   $ 281,000,000