Debt - Components of Debt (Parenthetical) (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2017	Dec. 31, 2016
Capital Leases and Other [Member]
Debt Instrument [Line Items]
End period of maturity for debt instrument	Dec. 31, 2055
Commercial Paper [Member]
Debt Instrument [Line Items]
Weighted average interest rate	1.20%
Senior Notes [Member]
Debt Instrument [Line Items]
Weighted average interest rate	4.60%	4.60%
End period of maturity for debt instrument	Mar. 01, 2045
Tax-exempt Bonds [Member]
Debt Instrument [Line Items]
Weighted average interest rate	1.90%	1.80%
End period of maturity for debt instrument	Aug. 01, 2045
Minimum [Member] | Senior Notes [Member]
Debt Instrument [Line Items]
Interest rate	2.40%
Minimum [Member] | Tax-exempt Bonds [Member]
Debt Instrument [Line Items]
Interest rate	0.90%
Maximum [Member] | Capital Leases and Other [Member]
Debt Instrument [Line Items]
Interest rate	12.00%
Maximum [Member] | Senior Notes [Member]
Debt Instrument [Line Items]
Interest rate	7.75%
Maximum [Member] | Tax-exempt Bonds [Member]
Debt Instrument [Line Items]
Interest rate	5.70%
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Credit Facility, aggregate capacity	$ 2,250,000,000	$ 2,250,000,000
Weighted average interest rate		1.90%
Maturity date of credit facility	Jul. 30, 2020
Other Letter of Credit Facilities [Member]
Debt Instrument [Line Items]
Maturity date of credit facility	Dec. 31, 2018
Canadian Term Loan and Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Weighted average interest rate	2.10%	2.10%
Maturity date of credit facility	Mar. 31, 2019