Fair Value Measurement - Fair Value of Financial Instruments (Details) - USD ($) $ in Millions		Sep. 30, 2015	Dec. 31, 2014
Assets:
Fixed-maturity securities		$ 11,162	$ 11,258
Short-term investments		522	767
Other invested assets		181	126
Credit derivative assets		71	68
Liabilities:
Credit derivative liabilities		918	963
Carrying Amount [Member]
Assets:
Fixed-maturity securities		10,640	10,491
Short-term investments		522	767
Other invested assets		153	108
Credit derivative assets		71	68
FG VIEs’ assets, at fair value		1,541	1,398
Other assets		194	184
Liabilities:
Financial guaranty insurance contracts	[1]	4,019	3,823
Long-term debt		1,306	1,303
Credit derivative liabilities		918	963
FG VIEs’ liabilities with recourse, at fair value		1,315	1,277
FG VIEs’ liabilities without recourse, at fair value		167	142
Other liabilities		65	27
Estimated Fair Value [Member]
Assets:
Fixed-maturity securities		10,640	10,491
Short-term investments		522	767
Other invested assets		155	110
Credit derivative assets		71	68
FG VIEs’ assets, at fair value		1,541	1,398
Other assets		194	184
Liabilities:
Financial guaranty insurance contracts	[1]	8,807	6,205
Long-term debt		1,513	1,603
Credit derivative liabilities		918	963
FG VIEs’ liabilities with recourse, at fair value		1,315	1,277
FG VIEs’ liabilities without recourse, at fair value		167	142
Other liabilities		$ 65	$ 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.