Investments (details) - Gross Realized Gains (Losses) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Fixed maturities [Member]
Gross realized gains (losses)
Gross realized gains	$ 70	$ 63	$ 106
Gross realized losses	9	10	11
Gross realized losses
Equity securities [Member]
Gross realized gains (losses)
Gross realized gains	8	48	128
Gross realized losses		2
Gross realized losses	less than $1 million		less than $1 million
Real estate [Member]
Gross realized gains (losses)
Gross realized gains	19		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