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New Accounting Standards and Reclassifications (Detail) - USD ($)
$ in Millions
Jan. 01, 2018
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
New ASU          
Restricted trust and escrow accounts   $ 296 $ 249    
Accounting Standards Update 2016-18 [Member]          
New ASU          
Restricted cash and cash equivalents     $ 271    
Restricted trust and escrow accounts       $ 62 $ 65
Accounting Standards Update 2018-02 [Member]          
New ASU          
Adoption of new accounting standard $ 5        
Accounting Standards Update 2016-02 [Member] | Scenario Forecast Adjustment [Member] | Minimum [Member]          
New ASU          
Right-of-use assets   300      
Operating lease, liability   300      
Accounting Standards Update 2016-02 [Member] | Scenario Forecast Adjustment [Member] | Maximum [Member]          
New ASU          
Right-of-use assets   350      
Operating lease, liability   $ 350