Benefit Plans - Other Postretirement Benefit Plan Assets Measured at Fair Value on Recurring Basis (Detail) - Other Postretirement Benefits [Member] - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 85	$ 88	$ 86
Corporate Bonds and Other [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	24	18
States, Municipalities and Political Subdivisions [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	11	42
Total Asset-Backed [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	30	12
Fixed Maturity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	65	72
Short Term and Other [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4	2
Fixed Income Mutual Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	16	14
Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	20	16
Level 1 [Member] | Short Term and Other [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4	2
Level 1 [Member] | Fixed Income Mutual Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	16	14
Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	65	72
Level 2 [Member] | Corporate Bonds and Other [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	24	18
Level 2 [Member] | States, Municipalities and Political Subdivisions [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	11	42
Level 2 [Member] | Total Asset-Backed [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	30	12
Level 2 [Member] | Fixed Maturity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 65	$ 72