Financial Guarantee Insurance Contracts Financial Guarantee Insurance Contracts Earned Premiums by Geographic Location (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Premiums Earned, Net, Financial Guarantee Insurance Contracts	$ 54	$ 66	$ 111
Premiums Receivable, Gross	387
Public Finance [Member]
Premiums Receivable, Gross	185
Structured Finance [Member]
Premiums Receivable, Gross	59
International Finance [Member]
Premiums Receivable, Gross	144
Housing Revenue [Member] | Public Finance [Member]
Premiums Receivable, Gross	168
Other Public Finance Sectors [Member] | Public Finance [Member]
Premiums Receivable, Gross	17
Mortgage-backed and Home Equity [Member] | Structured Finance [Member]
Premiums Receivable, Gross	22
Student Loan [Member] | Structured Finance [Member]
Premiums Receivable, Gross	16
Structured Insurance [Member] | Structured Finance [Member]
Premiums Receivable, Gross	14
Other Structured Finance [Member] | Structured Finance [Member]
Premiums Receivable, Gross	7
Sovereign/Sub-sovereign [Member] | International Finance [Member]
Premiums Receivable, Gross	108
Investor-owned and Public Utilities [Member] | International Finance [Member]
Premiums Receivable, Gross	31
Other International [Member] | International Finance [Member]
Premiums Receivable, Gross	5
Surveillance Category One [Member]
Premiums Receivable, Gross	302
Surveillance Category One [Member] | Public Finance [Member]
Premiums Receivable, Gross	157
Surveillance Category One [Member] | Structured Finance [Member]
Premiums Receivable, Gross	27
Surveillance Category One [Member] | International Finance [Member]
Premiums Receivable, Gross	118
Surveillance Category One [Member] | Housing Revenue [Member] | Public Finance [Member]
Premiums Receivable, Gross	155
Surveillance Category One [Member] | Other Public Finance Sectors [Member] | Public Finance [Member]
Premiums Receivable, Gross	2
Surveillance Category One [Member] | Mortgage-backed and Home Equity [Member] | Structured Finance [Member]
Premiums Receivable, Gross	3
Surveillance Category One [Member] | Student Loan [Member] | Structured Finance [Member]
Premiums Receivable, Gross	3
Surveillance Category One [Member] | Structured Insurance [Member] | Structured Finance [Member]
Premiums Receivable, Gross	14
Surveillance Category One [Member] | Other Structured Finance [Member] | Structured Finance [Member]
Premiums Receivable, Gross	7
Surveillance Category One [Member] | Sovereign/Sub-sovereign [Member] | International Finance [Member]
Premiums Receivable, Gross	82
Surveillance Category One [Member] | Investor-owned and Public Utilities [Member] | International Finance [Member]
Premiums Receivable, Gross	31
Surveillance Category One [Member] | Other International [Member] | International Finance [Member]
Premiums Receivable, Gross	5
IA [Member]
Premiums Receivable, Gross	40
IA [Member] | Public Finance [Member]
Premiums Receivable, Gross	27
IA [Member] | Structured Finance [Member]
Premiums Receivable, Gross	0
IA [Member] | International Finance [Member]
Premiums Receivable, Gross	13
IA [Member] | Housing Revenue [Member] | Public Finance [Member]
Premiums Receivable, Gross	13
IA [Member] | Other Public Finance Sectors [Member] | Public Finance [Member]
Premiums Receivable, Gross	15
IA [Member] | Mortgage-backed and Home Equity [Member] | Structured Finance [Member]
Premiums Receivable, Gross	0
IA [Member] | Student Loan [Member] | Structured Finance [Member]
Premiums Receivable, Gross	0
IA [Member] | Structured Insurance [Member] | Structured Finance [Member]
Premiums Receivable, Gross	0
IA [Member] | Other Structured Finance [Member] | Structured Finance [Member]
Premiums Receivable, Gross	0
IA [Member] | Sovereign/Sub-sovereign [Member] | International Finance [Member]
Premiums Receivable, Gross	13
IA [Member] | Investor-owned and Public Utilities [Member] | International Finance [Member]
Premiums Receivable, Gross	0
IA [Member] | Other International [Member] | International Finance [Member]
Premiums Receivable, Gross	0
II [Member]
Premiums Receivable, Gross	3
II [Member] | Public Finance [Member]
Premiums Receivable, Gross	0
II [Member] | Structured Finance [Member]
Premiums Receivable, Gross	3
II [Member] | International Finance [Member]
Premiums Receivable, Gross	0
II [Member] | Housing Revenue [Member] | Public Finance [Member]
Premiums Receivable, Gross	0
II [Member] | Other Public Finance Sectors [Member] | Public Finance [Member]
Premiums Receivable, Gross	0
II [Member] | Mortgage-backed and Home Equity [Member] | Structured Finance [Member]
Premiums Receivable, Gross	1
II [Member] | Student Loan [Member] | Structured Finance [Member]
Premiums Receivable, Gross	2
II [Member] | Structured Insurance [Member] | Structured Finance [Member]
Premiums Receivable, Gross	0
II [Member] | Other Structured Finance [Member] | Structured Finance [Member]
Premiums Receivable, Gross	0
II [Member] | Sovereign/Sub-sovereign [Member] | International Finance [Member]
Premiums Receivable, Gross	0
II [Member] | Investor-owned and Public Utilities [Member] | International Finance [Member]
Premiums Receivable, Gross	0
II [Member] | Other International [Member] | International Finance [Member]
Premiums Receivable, Gross	0
III [Member]
Premiums Receivable, Gross	27
III [Member] | Public Finance [Member]
Premiums Receivable, Gross	0
III [Member] | Structured Finance [Member]
Premiums Receivable, Gross	14
III [Member] | International Finance [Member]
Premiums Receivable, Gross	13
III [Member] | Housing Revenue [Member] | Public Finance [Member]
Premiums Receivable, Gross	0
III [Member] | Other Public Finance Sectors [Member] | Public Finance [Member]
Premiums Receivable, Gross	0
III [Member] | Mortgage-backed and Home Equity [Member] | Structured Finance [Member]
Premiums Receivable, Gross	3
III [Member] | Student Loan [Member] | Structured Finance [Member]
Premiums Receivable, Gross	11
III [Member] | Structured Insurance [Member] | Structured Finance [Member]
Premiums Receivable, Gross	0
III [Member] | Other Structured Finance [Member] | Structured Finance [Member]
Premiums Receivable, Gross	0
III [Member] | Sovereign/Sub-sovereign [Member] | International Finance [Member]
Premiums Receivable, Gross	13
III [Member] | Investor-owned and Public Utilities [Member] | International Finance [Member]
Premiums Receivable, Gross	0
III [Member] | Other International [Member] | International Finance [Member]
Premiums Receivable, Gross	0
IV [Member]
Premiums Receivable, Gross	15
IV [Member] | Public Finance [Member]
Premiums Receivable, Gross	0
IV [Member] | Structured Finance [Member]
Premiums Receivable, Gross	15
IV [Member] | International Finance [Member]
Premiums Receivable, Gross	0
IV [Member] | Housing Revenue [Member] | Public Finance [Member]
Premiums Receivable, Gross	0
IV [Member] | Other Public Finance Sectors [Member] | Public Finance [Member]
Premiums Receivable, Gross	0
IV [Member] | Mortgage-backed and Home Equity [Member] | Structured Finance [Member]
Premiums Receivable, Gross	15
IV [Member] | Student Loan [Member] | Structured Finance [Member]
Premiums Receivable, Gross	0
IV [Member] | Structured Insurance [Member] | Structured Finance [Member]
Premiums Receivable, Gross	0
IV [Member] | Other Structured Finance [Member] | Structured Finance [Member]
Premiums Receivable, Gross	0
IV [Member] | Sovereign/Sub-sovereign [Member] | International Finance [Member]
Premiums Receivable, Gross	0
IV [Member] | Investor-owned and Public Utilities [Member] | International Finance [Member]
Premiums Receivable, Gross	0
IV [Member] | Other International [Member] | International Finance [Member]
Premiums Receivable, Gross	0
Reportable Geographical Components [Member] | UNITED STATES
Premiums Earned, Net, Financial Guarantee Insurance Contracts	32	55	88
Reportable Geographical Components [Member] | UNITED KINGDOM
Premiums Earned, Net, Financial Guarantee Insurance Contracts	24	17	19
Reportable Geographical Components [Member] | Other International [Member]
Premiums Earned, Net, Financial Guarantee Insurance Contracts	$ (2)	$ (6)	$ 5