Benefit Plans - Pension Plan Assets Measured at Fair Value on Recurring Basis (Detail) - Pension Benefits [Member] - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Total plan assets	$ 2,576	$ 2,304	$ 2,577
Corporate Bonds and Other [Member]
Defined Benefit Plan Disclosure [Line Items]
Total plan assets	597	482
States, Municipalities and Political Subdivisions [Member]
Defined Benefit Plan Disclosure [Line Items]
Total plan assets	51	58
Total Asset-Backed [Member]
Defined Benefit Plan Disclosure [Line Items]
Total plan assets	154	165
Fixed Maturity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Total plan assets	802	705
Defined Benefit Plan, Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Total plan assets	669	516
Short-term Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Total plan assets	81	90
Fixed Income Mutual Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Total plan assets	128	120
Other Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Total plan assets	20	9
Fair Value of Plan Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Total plan assets	1,700	1,440
Limited Partnership Investment [Member]
Defined Benefit Plan Disclosure [Line Items]
Total plan assets	876	864
Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total plan assets	754	562
Level 1 [Member] | Defined Benefit Plan, Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Total plan assets	541	406
Level 1 [Member] | Short-term Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Total plan assets	74	36
Level 1 [Member] | Fixed Income Mutual Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Total plan assets	128	120
Level 1 [Member] | Other Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Total plan assets	11
Level 1 [Member] | Fair Value of Plan Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Total plan assets	754	562
Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total plan assets	936	868
Level 2 [Member] | Corporate Bonds and Other [Member]
Defined Benefit Plan Disclosure [Line Items]
Total plan assets	587	472
Level 2 [Member] | States, Municipalities and Political Subdivisions [Member]
Defined Benefit Plan Disclosure [Line Items]
Total plan assets	51	58
Level 2 [Member] | Total Asset-Backed [Member]
Defined Benefit Plan Disclosure [Line Items]
Total plan assets	154	165
Level 2 [Member] | Fixed Maturity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Total plan assets	792	695
Level 2 [Member] | Defined Benefit Plan, Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Total plan assets	128	110
Level 2 [Member] | Short-term Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Total plan assets	7	54
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	936	868
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