Investments (details) - Gross Realized Gains (Losses) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Fixed maturities [Member]
Gross realized gains (losses)
Gross realized gains	$ 63	$ 106	$ 119
Gross realized losses	10	11	19
Equity securities [Member]
Gross realized gains (losses)
Gross realized gains	48	128	13
Gross realized losses	2		2
Gross realized losses		less than $1 million
Real estate [Member]
Gross realized gains (losses)
Gross realized gains		3
Equity securities, investment in Verisk [Member]
Gross realized gains (losses)
Gross realized gains		102
Other investments, investment in Verisk [Member]
Gross realized gains (losses)
Gross realized gains		$ 103	$ 159