v3.20.4
Postretirement Benefit Plans - Classification of Pension Plan Assets (Detail) - USD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Defined Benefit Plan, Plan Assets, Category [Line Items]    
Fair value of plan assets $ 224  
Level 1    
Defined Benefit Plan, Plan Assets, Category [Line Items]    
Fair value of plan assets 8 $ 13
Level 2    
Defined Benefit Plan, Plan Assets, Category [Line Items]    
Fair value of plan assets 169 153
Level III    
Defined Benefit Plan, Plan Assets, Category [Line Items]    
Fair value of plan assets 18 14
Equity securities | Level 1    
Defined Benefit Plan, Plan Assets, Category [Line Items]    
Fair value of plan assets 0 0
Equity securities | Level 2    
Defined Benefit Plan, Plan Assets, Category [Line Items]    
Fair value of plan assets 71 59
Equity securities | Level III    
Defined Benefit Plan, Plan Assets, Category [Line Items]    
Fair value of plan assets 0 0
Debt securities | Level 1    
Defined Benefit Plan, Plan Assets, Category [Line Items]    
Fair value of plan assets 8 11
Debt securities | Level 2    
Defined Benefit Plan, Plan Assets, Category [Line Items]    
Fair value of plan assets 62 62
Debt securities | Level III    
Defined Benefit Plan, Plan Assets, Category [Line Items]    
Fair value of plan assets 0 0
Insurance contracts | Level 1    
Defined Benefit Plan, Plan Assets, Category [Line Items]    
Fair value of plan assets 0 0
Insurance contracts | Level 2    
Defined Benefit Plan, Plan Assets, Category [Line Items]    
Fair value of plan assets 15 14
Insurance contracts | Level III    
Defined Benefit Plan, Plan Assets, Category [Line Items]    
Fair value of plan assets 0 0
Other | Level 1    
Defined Benefit Plan, Plan Assets, Category [Line Items]    
Fair value of plan assets 0 2
Other | Level 2    
Defined Benefit Plan, Plan Assets, Category [Line Items]    
Fair value of plan assets 21 18
Other | Level III    
Defined Benefit Plan, Plan Assets, Category [Line Items]    
Fair value of plan assets $ 18 $ 14