Benefit Plans - Other Postretirement Benefit Plan Assets Measured at Fair Value on Recurring Basis (Detail) (Other Postretirement Benefits [Member], USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 87	$ 81	$ 87
Corporate and Other Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	18	17
States, Municipalities and Political Subdivisions [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	43	38
Total Asset-Backed [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	20	20
Total Fixed Maturities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	81	75
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	3	3
Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	6	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	3	3
Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	81	75
Level 2 [Member] | Corporate and Other Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	18	17
Level 2 [Member] | States, Municipalities and Political Subdivisions [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	43	38
Level 2 [Member] | Total Asset-Backed [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	20	20
Level 2 [Member] | Total Fixed Maturities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 81	$ 75