Investments and Cash (Details 5) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Schedule of Investments [Line Items]
Fair value of the securities purchased to mitigate insured losses	$ 853	$ 670
Obligations of state and political subdivisions
Schedule of Investments [Line Items]
Fair value of the securities purchased to mitigate insured losses	41	35
Corporate securities
Schedule of Investments [Line Items]
Fair value of the securities purchased to mitigate insured losses	132
RMBS
Schedule of Investments [Line Items]
Fair value of the securities purchased to mitigate insured losses	268	215
Asset-backed securities
Schedule of Investments [Line Items]
Fair value of the securities purchased to mitigate insured losses	340	306
Assets Acquired in Refinancing Transactions
Schedule of Investments [Line Items]
Fair value of the securities purchased to mitigate insured losses	51	72
Other asset
Schedule of Investments [Line Items]
Fair value of the securities purchased to mitigate insured losses	$ 21	$ 42