Benefit Plans - Pension Plan Assets Measured at Fair Value on Recurring Basis (Detail) - Pension Benefits [Member] - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 2,423	$ 2,500	$ 2,713
Corporate and Other Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	510	465
States, Municipalities and Political Subdivisions [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	63	106
Total Asset-Backed [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	186	219
Total Fixed Maturities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	759	790
Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	509	480
Short Term Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	53	58
Fixed Income Mutual Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	92	95
Other Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	52	52
Limited Partnerships [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	958	1,025
Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	529	498
Level 1 [Member] | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	404	373
Level 1 [Member] | Short Term Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	18	30
Level 1 [Member] | Fixed Income Mutual Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	92	95
Level 1 [Member] | Other Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	15
Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	926	967
Level 2 [Member] | Corporate and Other Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	500	455
Level 2 [Member] | States, Municipalities and Political Subdivisions [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	63	106
Level 2 [Member] | Total Asset-Backed [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	186	219
Level 2 [Member] | Total Fixed Maturities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	749	780
Level 2 [Member] | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	105	107
Level 2 [Member] | Short Term Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	35	28
Level 2 [Member] | Other Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	37	52
Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	10	10	15
Level 3 [Member] | Corporate and Other Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	10	10	$ 15
Level 3 [Member] | Total Fixed Maturities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 10	$ 10