Investments (Net investment income) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Gross investment income	$ 2,338	$ 2,112	$ 2,370
Investment expense	(56)	(58)	(54)
Net investment income	2,282	2,054	2,316
Fixed maturity securities [Member]
Gross investment income	2,022	2,011	2,051
Short term investments [Member]
Gross investment income	5	8	15
Limited partnership investments [Member]
Gross investment income	251	48	249
Equity securities [Member]
Gross investment income	12	20	32
Mortgage loans [Member]
Gross investment income	17	9	2
Trading portfolio [Member]
Gross investment income	24	9	13
Other [Member]
Gross investment income	$ 7	$ 7	$ 8