Fair Value Measurement - Fair Value of Financial Instruments (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015		Dec. 31, 2014
Assets:
Fixed-maturity securities	$ 10,182		$ 11,258
Short-term investments	349		767
Other invested assets	132		126
Credit derivative assets	77		68
Liabilities:
Credit derivative liabilities	859		963
Carrying Amount [Member]
Assets:
Fixed-maturity securities	9,833		10,491
Short-term investments	349		767
Other invested assets	114		108
Credit derivative assets	77		68
FG VIEs’ assets, at fair value	1,495		1,398
Other assets	189		184
Liabilities:
Financial guaranty insurance contracts	3,719	[1]	3,823	[1]
Long-term debt	1,304		1,303
Credit derivative liabilities	859		963
FG VIEs’ liabilities with recourse, at fair value	1,278		1,277
FG VIEs’ liabilities without recourse, at fair value	145		142
Other liabilities	72		27
Estimated Fair Value [Member]
Assets:
Fixed-maturity securities	9,833		10,491
Short-term investments	349		767
Other invested assets	116		110
Credit derivative assets	77		68
FG VIEs’ assets, at fair value	1,495		1,398
Other assets	189		184
Liabilities:
Financial guaranty insurance contracts	6,343	[1]	6,205	[1]
Long-term debt	1,609		1,603
Credit derivative liabilities	859		963
FG VIEs’ liabilities with recourse, at fair value	1,278		1,277
FG VIEs’ liabilities without recourse, at fair value	145		142
Other liabilities	$ 72		$ 27

[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.