Fair Value Measurement (Details 5) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013		Dec. 31, 2012
Assets:
Short-term investments	$ 729		$ 817
Other invested assets	148		212
Credit derivative assets	125		141
FG VIEs' assets, at fair value	2,813		2,688
Liabilities:
Credit derivative liabilities	2,518		1,934
FG VIEs' liabilities with recourse, at fair value	2,071		2,090
FG VIEs' liabilities without recourse, at fair value	1,107		1,051
Carrying Amount
Assets:
Fixed maturity securities	9,985		10,056
Short-term investments	729		817
Other invested assets	119		177
Credit derivative assets	125		141
FG VIEs' assets, at fair value	2,813		2,688
Other assets	210		166
Liabilities:
Financial guaranty insurance contracts	3,591	[1]	3,918	[1]
Long-term debt	832		836
Credit derivative liabilities	2,518		1,934
FG VIEs' liabilities with recourse, at fair value	2,071		2,090
FG VIEs' liabilities without recourse, at fair value	1,107		1,051
Other liabilities	88		47
Estimated Fair Value
Assets:
Fixed maturity securities	9,985		10,056
Short-term investments	729		817
Other invested assets	122		182
Credit derivative assets	125		141
FG VIEs' assets, at fair value	2,813		2,688
Other assets	210		166
Liabilities:
Financial guaranty insurance contracts	5,955	[1]	6,537	[1]
Long-term debt	1,131		1,091
Credit derivative liabilities	2,518		1,934
FG VIEs' liabilities with recourse, at fair value	2,071		2,090
FG VIEs' liabilities without recourse, at fair value	1,107		1,051
Other liabilities	$ 88		$ 47

[1] Carrying amount includes the assets and liabilities related to financial guaranty insurance contract premiums, losses, and salvage and subrogation and other recoverables net of reinsurance.