Fair Values of Financial Instruments - Fair Values of Pension Plan Assets Categorized Based Upon Lowest Level of Input (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
Other Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	54	47
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
Fixed maturities [Member] | U.S. government and government agency and authority obligations [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	239	207
Fixed maturities [Member] | Corporate bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	321	290
Fixed maturities [Member] | Foreign government and government agency obligations [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	80	62
Fixed maturities [Member] | Mortgage Backed Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	174	176
Equity securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	1,404	1,174
Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	486	357
Level 1 [Member] | Other Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	29	21
Level 1 [Member] | Short term investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value
Level 1 [Member] | Fixed maturities [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value
Level 1 [Member] | Fixed maturities [Member] | U.S. government and government agency and authority obligations [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value
Level 1 [Member] | Fixed maturities [Member] | Corporate bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value
Level 1 [Member] | Fixed maturities [Member] | Foreign government and government agency obligations [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value
Level 1 [Member] | Fixed maturities [Member] | Mortgage Backed Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value
Level 1 [Member] | Equity securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	457	336
Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	1,801	1,620
Level 2 [Member] | Other Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	9	8
Level 2 [Member] | Short term investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	33	45
Level 2 [Member] | Fixed maturities [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	812	729
Level 2 [Member] | Fixed maturities [Member] | U.S. government and government agency and authority obligations [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	238	204
Level 2 [Member] | Fixed maturities [Member] | Corporate bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	321	289
Level 2 [Member] | Fixed maturities [Member] | Foreign government and government agency obligations [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	79	61
Level 2 [Member] | Fixed maturities [Member] | Mortgage Backed Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	174	175
Level 2 [Member] | Equity securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	947	838
Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	18	24
Level 3 [Member] | Other Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	16	18
Level 3 [Member] | Short term investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value
Level 3 [Member] | Fixed maturities [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	2	6
Level 3 [Member] | Fixed maturities [Member] | U.S. government and government agency and authority obligations [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	1	3
Level 3 [Member] | Fixed maturities [Member] | Corporate bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value		1
Level 3 [Member] | Fixed maturities [Member] | Foreign government and government agency obligations [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	1	1
Level 3 [Member] | Fixed maturities [Member] | Mortgage Backed Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value		1
Level 3 [Member] | Equity securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value