Debt - Components of Debt (Parenthetical) (Detail)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Senior Notes [Member]
Debt Instrument [Line Items]
Weighted average interest rate	4.70%	5.70%
Interest rate lower range	2.60%
Interest rate upper range	7.75%
End period of maturity for debt instrument	Mar. 01, 2045
Tax-exempt Bonds [Member]
Debt Instrument [Line Items]
Weighted average interest rate	1.90%	2.20%
Interest rate lower range	0.02%
Interest rate upper range	5.70%
End period of maturity for debt instrument	Aug. 01, 2045
Capital Leases and Other [Member]
Debt Instrument [Line Items]
Interest rate upper range	12.00%
End period of maturity for debt instrument	Dec. 31, 2055
U.S. Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Maturity date of credit facility	Jul. 10, 2020
Standby Letters of Credit [Member]
Debt Instrument [Line Items]
Maturity date of credit facility	Dec. 31, 2018
Canadian Credit Facility and Term Loan [Member]
Debt Instrument [Line Items]
Weighted average interest rate	2.20%	2.60%
Maturity date of credit facility	Nov. 07, 2017