Employee Benefits - Fair Values of Pension Plan Assets (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	$ 2,305	$ 2,001
Short term investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	33	45
Fixed maturities [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	814	735
Equity securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	1,404	1,174
U.S. government and government agency and authority obligations [Member] | Fixed maturities [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	239	207
Corporate bonds [Member] | Fixed maturities [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	321	290
Foreign government and government agency obligations [Member] | Fixed maturities [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	80	62
Mortgage Backed Securities [Member] | Fixed maturities [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	174	176
Other Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	$ 54	$ 47