Variable Interest Entities (Detail) $ in Millions	6 Months Ended
	Jun. 30, 2018 USD ($) item	Dec. 31, 2017 USD ($)
Variable Interest Entity, Not Primary Beneficiary [Member] | Other Receivables Investments in Unconsolidated Entities and Other Assets [Member]
Variable Interest Entity [Line Items]
Aggregate investment balance	$ 99	$ 99
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
Aggregate investment balance	$ 48	59
Investment In Low Income Housing Properties [Member] | Variable Interest Entity, Not Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
Equity method investments debt balance	23	34
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	$ 102	$ 101