Variable Interest Entities (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) item	Dec. 31, 2016 USD ($) item
Variable Interest Entity [Line Items]
Aggregate investment balance	$ 127	$ 173
Variable Interest Entity, Not Primary Beneficiary [Member] | Other Receivables Investments in Unconsolidated Entities and Other Assets [Member]
Variable Interest Entity [Line Items]
Value of unconsolidated VIEs	$ 99	$ 93
Investments Qualifying for Federal Tax Credits [Member] | Variable Interest Entity, Not Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
Number of variable interest entities | item	2	2
Aggregate investment balance	$ 59	$ 84
Investment In Low Income Housing Properties [Member] | Variable Interest Entity, Not Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
Equity method investments debt balance	34	57
Trust For Final Capping, Closure, Post-closure Or Environmental Remediation Obligations [Member] | Variable Interest Entity, Primary Beneficiary [Member] | Long-term other assets [Member]
Variable Interest Entity [Line Items]
Value of consolidated VIEs	$ 101	$ 95