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Employee Bargaining Agreements and Benefit Plans - Plan's Assets Carried at Fair Value (Detail) - USD ($)
$ in Millions
Dec. 31, 2018
Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]    
Total fair value measurements $ 15 $ 18
Equity Securities [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Total fair value measurements 5 10
Fixed Income Securities [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Total fair value measurements 5 3
Other [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Total fair value measurements 5 5
Level 1 [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Total fair value measurements 10 13
Level 1 [Member] | Equity Securities [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Total fair value measurements 5 10
Level 1 [Member] | Fixed Income Securities [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Total fair value measurements 5 3
Level 2 [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Total fair value measurements 5 5
Level 2 [Member] | Other [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Total fair value measurements $ 5 $ 5