Benefit Plans - Pension Plan Assets Measured at Fair Value on Recurring Basis (Detail) - Pension Benefits [Member] - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Total plan assets	$ 2,304	$ 2,577	$ 2,423
Corporate Bonds and Other [Member]
Defined Benefit Plan Disclosure [Line Items]
Total plan assets	482	532
States, Municipalities and Political Subdivisions [Member]
Defined Benefit Plan Disclosure [Line Items]
Total plan assets	58	62
Total Asset-Backed [Member]
Defined Benefit Plan Disclosure [Line Items]
Total plan assets	165	182
Fixed Maturity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Total plan assets	705	776
Defined Benefit Plan, Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Total plan assets	516	571
Short Term and Other [Member]
Defined Benefit Plan Disclosure [Line Items]
Total plan assets	90	40
Fixed Income Mutual Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Total plan assets	120	96
Other Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Total plan assets	9	22
Fair Value of Plan Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Total plan assets	1,440	1,505
Limited Partnership Investment [Member]
Defined Benefit Plan Disclosure [Line Items]
Total plan assets	864	990
Collective Investment Trust Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Total plan assets		82
Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total plan assets	562	587
Level 1 [Member] | Defined Benefit Plan, Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Total plan assets	406	449
Level 1 [Member] | Short Term and Other [Member]
Defined Benefit Plan Disclosure [Line Items]
Total plan assets	36	29
Level 1 [Member] | Fixed Income Mutual Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Total plan assets	120	96
Level 1 [Member] | Other Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Total plan assets		13
Level 1 [Member] | Fair Value of Plan Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Total plan assets	562	587
Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total plan assets	868	908
Level 2 [Member] | Corporate Bonds and Other [Member]
Defined Benefit Plan Disclosure [Line Items]
Total plan assets	472	522
Level 2 [Member] | States, Municipalities and Political Subdivisions [Member]
Defined Benefit Plan Disclosure [Line Items]
Total plan assets	58	62
Level 2 [Member] | Total Asset-Backed [Member]
Defined Benefit Plan Disclosure [Line Items]
Total plan assets	165	182
Level 2 [Member] | Fixed Maturity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Total plan assets	695	766
Level 2 [Member] | Defined Benefit Plan, Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Total plan assets	110	122
Level 2 [Member] | Short Term and Other [Member]
Defined Benefit Plan Disclosure [Line Items]
Total plan assets	54	11
Level 2 [Member] | Other Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Total plan assets	9	9
Level 2 [Member] | Fair Value of Plan Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Total plan assets	868	908
Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total plan assets	10	10
Level 3 [Member] | Corporate Bonds and Other [Member]
Defined Benefit Plan Disclosure [Line Items]
Total plan assets	10	10
Level 3 [Member] | Fixed Maturity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Total plan assets	10	10
Level 3 [Member] | Fair Value of Plan Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Total plan assets	$ 10	$ 10