Investments (details) - Changes in Net Unrealized Investment Gains - Rollforward (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Changes in net unrealized investment gains [Roll Forward]
Changes in net unrealized investment gains	$ 362	$ 1,572	$ 5
Changes in net unrealized investment gains, tax expense	130	560	2
Change in net unrealized gain on investment securities, net of tax	232	1,012	3
Net unrealized gains on investment securities, balance, beginning of year	2,871	1,859	1,856
Net unrealized gains on investment securities, balance, end of year	$ 3,103	$ 2,871	$ 1,859