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Variable Interest Entities (Detail) - USD ($)
$ in Millions
Sep. 30, 2018
Dec. 31, 2017
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]    
Aggregate investment balance 199 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 156 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 $ 103 $ 101