Benefit Plans - Other Postretirement Benefit Plan Assets Measured at Fair Value on Recurring Basis (Detail) - Other Postretirement Benefits [Member] - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 86	$ 87	$ 81
Corporate and Other Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	17	18
States, Municipalities and Political Subdivisions [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	42	43
Total Asset-Backed [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	19	20
Total Fixed Maturities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	78	81
Short Term Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3	3
Fixed Income Mutual Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5	3
Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	8	6
Level 1 [Member] | Short Term Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3	3
Level 1 [Member] | Fixed Income Mutual Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5	3
Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	78	81
Level 2 [Member] | Corporate and Other Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	17	18
Level 2 [Member] | States, Municipalities and Political Subdivisions [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	42	43
Level 2 [Member] | Total Asset-Backed [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	19	20
Level 2 [Member] | Total Fixed Maturities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 78	$ 81