Fair Values of Financial Instruments - Carrying Values and Fair Values of Financial Instruments (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short term investments	$ 2,002	$ 2,528
Fixed maturities	37,300	38,076
Equity securities	1,845	1,663
Other invested assets	1,756	1,954
Long Term Debt	3,300	3,575
Carrying Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short term investments	2,002	2,528
Fixed maturities	37,300	38,076
Equity securities	1,845	1,663
Other invested assets		46
Long Term Debt	3,300	3,575
Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short term investments	2,002	2,528
Fixed maturities	37,300	38,076
Equity securities	1,845	1,663
Other invested assets		46
Long Term Debt	$ 3,851	$ 4,372