Benefit Plans - Other Postretirement Benefit Plan Assets Measured at Fair Value on Recurring Basis (Detail) - Other Postretirement Benefits [Member] - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 90	$ 85	$ 88
Corporate Bonds and Other [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	22	24
States, Municipalities and Political Subdivisions [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	16	11
Total Asset-Backed [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	31	30
Fixed Maturity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	69	65
Short Term and Other [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3	4
Fixed Income Mutual Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	18	16
Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	21	20
Level 1 [Member] | Short Term and Other [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3	4
Level 1 [Member] | Fixed Income Mutual Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	18	16
Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	69	65
Level 2 [Member] | Corporate Bonds and Other [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	22	24
Level 2 [Member] | States, Municipalities and Political Subdivisions [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	16	11
Level 2 [Member] | Total Asset-Backed [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	31	30
Level 2 [Member] | Fixed Maturity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 69	$ 65