Assumption of Insured Portfolio and Business Combinations - Narrative (Details) - USD ($) $ in Millions			3 Months Ended		9 Months Ended
	Jun. 01, 2018	Jan. 10, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Business Acquisition [Line Items]
Total net exposure			$ 246,940		$ 246,940		$ 264,952
Financial Guaranty Contracts Accounted for as Credit Derivatives Outstanding Principal Amount			6,475		6,475		6,207
Cash			82	$ 72	82	$ 72	144	$ 118
Premiums receivable, net of commissions payable			916		916		915
Credit risk derivatives, at fair value, net			(237)		(237)		$ (269)
MBIA UK Insurance Limited [Member]
Business Acquisition [Line Items]
Bargain purchase gain		$ 56
Gain on settlement of pre-existing relationships		2
Subsidiaries [Member] | MBIA UK Insurance Limited [Member]
Business Acquisition [Line Items]
Cash acquired		23
Bond exchanged, fair value		334
MBIA UK Insurance Limited [Member]
Business Acquisition [Line Items]
Total net exposure		12,000
Gross Par Outstanding		$ 347
Reinsurance of SGI Insured Portfolio [Member]
Business Acquisition [Line Items]
Net expected loss to be paid on assumed portfolio activity					$ 131
Reinsurance of SGI Insured Portfolio [Member] | AGC [Member]
Business Acquisition [Line Items]
Reinsurance retention policy, quota share basis, percent	100.00%
Total net exposure	$ 12,000
Cash	363
Premiums receivable, net of commissions payable	61
Net expected loss to be paid on assumed portfolio activity	131		$ 0	$ 0		$ 0
Professional Fees	4
Reinsurance of SGI Insured Portfolio [Member] | AGC [Member] | Assumed [Member]
Business Acquisition [Line Items]
Cash	343
Premiums receivable, net of commissions payable	45
Credit risk derivatives, at fair value, net	17
United States [Member] | SGI Portfolio [Member] | Reinsurance of SGI Insured Portfolio [Member]
Business Acquisition [Line Items]
Financial Guaranty Contracts Accounted for as Credit Derivatives Outstanding Principal Amount	1,500
Credit risk derivatives, at fair value, net	$ 68