Benefit Plans - Pension Plan Assets Measured at Fair Value on Recurring Basis (Detail) (Pension 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	$ 2,713	$ 2,914	$ 2,672
Corporate and Other Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	478	520
States, Municipalities and Political Subdivisions [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	80	73
Total Asset-Backed [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	216	254
U.S. Treasury and Obligations of Government-Sponsored Enterprises [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	25
Total Fixed Maturities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	799	847
Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	550	652
Short Term Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	159	98
Fixed Income Mutual Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	99	100
Hedge Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	952	1,057
Private Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	123	125
Limited Partnership Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,075	1,182
Other Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	31	35
Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	615	676
Level 1 [Member] | U.S. Treasury and Obligations of Government-Sponsored Enterprises [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	25
Level 1 [Member] | Total Fixed Maturities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	25
Level 1 [Member] | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	432	527
Level 1 [Member] | Short Term Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	58	49
Level 1 [Member] | Fixed Income Mutual Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	99	100
Level 1 [Member] | Other Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1
Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,627	1,738
Level 2 [Member] | Corporate and Other Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	463	505
Level 2 [Member] | States, Municipalities and Political Subdivisions [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	80	73
Level 2 [Member] | Total Asset-Backed [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	216	254
Level 2 [Member] | Total Fixed Maturities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	759	832
Level 2 [Member] | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	118	117
Level 2 [Member] | Short Term Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	101	49
Level 2 [Member] | Hedge Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	619	705
Level 2 [Member] | Limited Partnership Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	619	705
Level 2 [Member] | Other Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	30	35
Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	471	500
Level 3 [Member] | Corporate and Other Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	15	15
Level 3 [Member] | Total Fixed Maturities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	15	15
Level 3 [Member] | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		8
Level 3 [Member] | Hedge Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	333	352
Level 3 [Member] | Private Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	123	125
Level 3 [Member] | Limited Partnership Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 456	$ 477