Debt - Components of Debt (Parenthetical) (Detail) $ in Millions	6 Months Ended
	Jun. 30, 2017 USD ($)	Jun. 30, 2017 CAD	Dec. 31, 2016 USD ($)
Domestic Line of Credit [Member]
Debt Instrument [Line Items]
Credit Facility, aggregate capacity | $	$ 2,250		$ 2,250
Weighted average interest rate			1.90%
Maturity date of credit facility	Jul. 30, 2020
Commercial Paper [Member]
Debt Instrument [Line Items]
Weighted average interest rate	1.40%	1.40%
Other Letter of Credit Facilities [Member]
Debt Instrument [Line Items]
Maturity date of credit facility	Dec. 31, 2018
Foreign Line of Credit [Member]
Debt Instrument [Line Items]
Credit Facility, aggregate capacity | CAD		CAD 50,000,000
Weighted average interest rate	2.00%	2.00%	2.10%
Maturity date of credit facility	Mar. 24, 2019
Senior Notes [Member]
Debt Instrument [Line Items]
Weighted average interest rate	4.60%	4.60%	4.60%
End period of maturity for debt instrument	Mar. 01, 2045
Senior Notes [Member] | Minimum [Member]
Debt Instrument [Line Items]
Interest rate	2.40%	2.40%
Senior Notes [Member] | Maximum [Member]
Debt Instrument [Line Items]
Interest rate	7.75%	7.75%
Tax-exempt Bonds [Member]
Debt Instrument [Line Items]
Weighted average interest rate	1.90%	1.90%	1.80%
End period of maturity for debt instrument	Aug. 01, 2045
Tax-exempt Bonds [Member] | Minimum [Member]
Debt Instrument [Line Items]
Interest rate	0.90%	0.90%
Tax-exempt Bonds [Member] | Maximum [Member]
Debt Instrument [Line Items]
Interest rate	5.70%	5.70%
Capital Leases and Other [Member]
Debt Instrument [Line Items]
End period of maturity for debt instrument	Dec. 31, 2055
Capital Leases and Other [Member] | Maximum [Member]
Debt Instrument [Line Items]
Interest rate	12.00%	12.00%