Amount, after premiums ceded to other entities and premiums assumed by the entity, of financial guarantee premiums earned.

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Name:	us-gaap_PremiumsEarnedNetFinancialGuaranteeInsuranceContracts
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

Financial Guarantee Insurance Contracts Financial Guarantee Insurance Contracts Earned Premiums by Geographic Location (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Premiums Earned, Net, Financial Guarantee Insurance Contracts	$ 20	$ 10	$ 8	$ 28	$ 29	$ 26	$ 26	$ 31	$ 66	$ 111	$ 175
Reportable Geographical Components [Member] \| UNITED STATES
Premiums Earned, Net, Financial Guarantee Insurance Contracts									55	88	134
Reportable Geographical Components [Member] \| UNITED KINGDOM
Premiums Earned, Net, Financial Guarantee Insurance Contracts									17	19	33
Reportable Geographical Components [Member] \| Other International [Member]
Premiums Earned, Net, Financial Guarantee Insurance Contracts									$ (6)	$ 5	$ 8