Investments (Net investment income) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012		Mar. 31, 2011
Net Investment Income
Gross investment income	$ 662		$ 635
Investment expense	(14)		(15)
Net investment income	648		620
Fixed maturity securities [Member]
Net Investment Income
Gross investment income	516		506
Short term investments [Member]
Net Investment Income
Gross investment income	1		2
Limited partnership investments [Member]
Net Investment Income
Gross investment income	130		114
Equity securities [Member]
Net Investment Income
Gross investment income	4		6
Mortgage loans [Member]
Net Investment Income
Gross investment income	3		2
Trading portfolio [Member]
Net Investment Income
Gross investment income	7	[1]	3	[1]
Other [Member]
Net Investment Income
Gross investment income	$ 1		$ 2

[1] There were no net unrealized gains (losses) related to changes in fair value of trading securities still held included in net investment income for the three months ended March 31, 2012 or 2011.