Investments (details) - Changes in Net Unrealized Investment Gains (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Changes in net unrealized investment gains
Changes in net unrealized investment gains	$ 362	$ 1,572	$ 5
Fixed maturities [Member]
Changes in net unrealized investment gains
Changes in net unrealized investment gains	326	1,588	114
Equity securities [Member]
Changes in net unrealized investment gains
Changes in net unrealized investment gains	38	(2)	69
Other investments [Member]
Changes in net unrealized investment gains
Changes in net unrealized investment gains	$ (2)	$ (14)	$ (178)