Investments and Cash Investments and Cash (Details 6) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Schedule of Cost-method Investments [Line Items]
Fair value of the securities purchased to mitigate insured losses	$ 629		$ 670
Other income	(14)	91
Obligations of state and political subdivisions
Schedule of Cost-method Investments [Line Items]
Fair value of the securities purchased to mitigate insured losses	35		35
RMBS
Schedule of Cost-method Investments [Line Items]
Fair value of the securities purchased to mitigate insured losses	216		215
Asset-backed securities
Schedule of Cost-method Investments [Line Items]
Fair value of the securities purchased to mitigate insured losses	286		306
Assets Acquired in Refinancing Transactions
Schedule of Cost-method Investments [Line Items]
Fair value of the securities purchased to mitigate insured losses	67		72
Other asset
Schedule of Cost-method Investments [Line Items]
Fair value of the securities purchased to mitigate insured losses	$ 25		$ 42