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Variable Interest Entities - Additional Information (Detail) - USD ($)
$ in Millions
1 Months Ended
Jan. 31, 2011
Apr. 30, 2010
Mar. 31, 2016
Dec. 31, 2015
Trust for Final Capping Closure Post Closure or Environmental Remediation Obligations [Member]        
Variable Interest Entity [Line Items]        
Variable interest entities primary beneficiary trust fair value assets     $ 94 $ 94
Carrying value of trusts for which company is not the sole beneficiary     93 93
Investment in Refined Coal Facility [Member]        
Variable Interest Entity [Line Items]        
Cash payment to acquire equity method investment $ 48      
Investment balance     32 29
Investment in Low-Income Housing Properties [Member]        
Variable Interest Entity [Line Items]        
Cash payment to acquire equity method investment   $ 6    
Investment balance     75 81
Consideration for investment   221    
Consideration for investment, notes payable   $ 215    
Equity method investments debt balance     $ 74 $ 80