Contracts Accounted for as Insurance - Gross Premium Receivable Net of Commissions on Assumed Business Roll Forward (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
	Jun. 01, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Gross Premium Receivable Net of Ceding Commissions [Roll Forward]
Beginning balance				$ 915	$ 576
Less: Non-financial guaranty insurance premium receivable				1	0
FG insurance premiums receivable				914	576
Premiums receivable from acquisitions (see Note 2)				0	270
Gross written premiums on new business, net of commissions				508	225
Gross premiums received, net of commissions				(477)	(216)
Adjustments:
Changes in the expected term				(2)	0
Accretion of discount, net of commissions on assumed business				5	13
Foreign exchange translation				(23)	54
Cancellation of assumed reinsurance				(10)	0
FG insurance premiums receivable		$ 915	$ 922	915	922
Non-financial guaranty insurance premium receivable		1	0	1	0
Ending balance		916	922	916	922
Foreign Currency Gain (Loss) [Member]
Adjustments:
Foreign exchange translation		(8)	19	(21)	52
Variable Interest Entity, Primary Beneficiary [Member]
Adjustments:
Ending balance		$ 9	$ 10	$ 9	$ 10
Reinsurance of SGI Insured Portfolio [Member]
Gross Premium Receivable Net of Ceding Commissions [Roll Forward]
Gross written premiums on new business, net of commissions	$ 330
Gross premiums received, net of commissions	$ 275