Debt - Additional Information (Detail)	1 Months Ended		3 Months Ended
	Aug. 31, 2016 USD ($)	Mar. 31, 2016 CAD	Mar. 31, 2017 USD ($)	Mar. 31, 2017 CAD	Mar. 31, 2016 USD ($)	Mar. 31, 2017 CAD	Dec. 31, 2016 USD ($)
Debt Instrument [Line Items]
Current portion of long-term debt			$ 396,000,000				$ 417,000,000
Commercial paper, borrowings			210,000,000
Debt maturing within twelve months			186,000,000
Net borrowings (repayments)			210,000,000
Foreign Currency Derivatives [Member] | Cash Flow Hedging [Member]
Debt Instrument [Line Items]
Cash received in termination of foreign exchange cross currency swaps					$ 67,000,000
Expense associated with termination of foreign exchange cross currency swaps					$ 8,000,000
Canadian Term Loan and Credit Facility [Member]
Debt Instrument [Line Items]
Credit Facility, aggregate capacity | CAD		CAD 50,000,000
Outstanding borrowings under credit facility | CAD						CAD 0
Letters of credit outstanding revolving credit facility | CAD						CAD 0
Maturity date of credit facility		Mar. 31, 2019
Tax-exempt Bonds [Member]
Debt Instrument [Line Items]
Debt maturing within twelve months			126,000,000
Tax-exempt bonds subject to re-pricing within the next 12 months			490,000,000
Tax-exempt Bonds [Member] | Variable Rate [Member]
Debt Instrument [Line Items]
Variable-rate tax-exempt bonds			471,000,000
6.10% Senior Notes Mature in March 2018 [Member]
Debt Instrument [Line Items]
Senior note amount intended to refinance on a long-term basis			$ 590,000,000
Interest rate			6.10%			6.10%
Maximum [Member] | Tax-exempt Bonds [Member]
Debt Instrument [Line Items]
Interest rate			5.70%			5.70%
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Credit Facility, aggregate capacity			$ 2,250,000,000				$ 2,250,000,000
Outstanding borrowings under credit facility			0
Letters of credit outstanding revolving credit facility			783,000,000
Unused and available credit capacity			$ 1,257,000,000
Maturity date of credit facility			Jul. 30, 2020	Jul. 30, 2020
Net borrowings (repayments)			$ 426,000,000
Non Revolving Term Credit Facility [Member] | 2016 Canadian Credit Agreement [Member]
Debt Instrument [Line Items]
Credit Facility, aggregate capacity | CAD		CAD 460,000,000
Committed and Uncommitted [Member]
Debt Instrument [Line Items]
Letters of credit outstanding revolving credit facility			457,000,000
Canadian Term Loan [Member]
Debt Instrument [Line Items]
Net borrowings (repayments)			36,000,000	CAD 48,000,000
Canadian Term Loan [Member] | Currency Swaps [Member]
Debt Instrument [Line Items]
Net amount of derivatives for intercompany debt CAD | CAD		CAD 370,000,000
Commercial Paper [Member] | Revolving Credit Facility [Member] | Commercial Paper Program [Member]
Debt Instrument [Line Items]
Commercial paper, borrowings			$ 210,000,000
Credit Facility, Borrowing Capacity	$ 1,500,000,000
Commercial Paper [Member] | Revolving Credit Facility [Member] | Maximum [Member]
Debt Instrument [Line Items]
Debt term	397 days