Financial Guarantee Insurance Contracts - Additional Information (Detail) £ in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020 USD ($)	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2020 GBP (£)	Jan. 01, 2020 USD ($)	Dec. 31, 2019 GBP (£)	Feb. 12, 2019	Dec. 31, 2018 GBP (£)	Feb. 12, 2018 USD ($)	Dec. 31, 2017 USD ($)
Insurance [Line Items]
Ceded Loss And Loss Expenses Paid Not Yet Recovered	$ 1				$ 0				$ 1	$ 0	$ 1
Net par amount of financial guarantees	33,888				38,018				33,888	38,018
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	1,140				$ 1,536				1,140	$ 1,536	1,633							$ 1,645
Stockholders Equity After Possible Increase in Domestic Public Finance Loss Reserves	(60)								(60)
Possible Increase in Domestic Public Finance Loss Reserves	$ 1,200								$ 1,200
Estimated Future Premium Payments Weighted Average Discounted Rate	2.20%				2.40%				2.20%	2.40%		2.20%		2.40%
Financial Guarantee Insurance Contracts, Premium Receivable	$ 370				$ 416				$ 370	$ 416	495							586
Amount Of Insured Par Outstanding Ceded To Reinsurer	2,398								$ 2,398
Ceded Principal Outstanding Major Reinsurer Percentage									6.10%
Deferred Ceded Premiums And Reinsurance Recoverables	103								$ 103
Reinsurance Payable	27				29				$ 27	$ 29
Weighted average period of future premiums									8 years 3 months 18 days	8 years 6 months
Uncollectable premium receivables	17								$ 17
Accelerated premium revenue for retired obligations									12	$ 10	32
Reinsurance recoveries of losses included in losses and loss expenses									(11)	(7)	(2)
Losses and loss expense reserves ceded to reinsurers	33				26				33	26	23							$ 41
Subrogation recoveries, net of reinsurance	(1,725)				(1,702)				(1,725)	(1,702)
Amortization of insurance intangible assets	16	$ 14	$ 14	$ 13	15	$ 17	$ 226	$ 36	57	295	107
Intangible Assets, Gross (Excluding Goodwill)	1,281				1,273				1,281	1,273
Finite-Lived Intangible Assets, Accumulated Amortization	908				847				908	847
Intangible assets	409				427				409	427
Letters of Credit Outstanding, Amount	117								117
Financial Guarantee Outstanding Principal Ceded To Third Parties	5,182								5,182
Public Finance Puerto Rico Net Par Outstanding	1,070								1,070
Percent Cash & New COFINA Bonds Allocated Under POA															93.00%
Ambac Assurance Remaining Policy Obligations	80								80
Premiums Receivable, Gross	387								387
Allowance for Doubtful Accounts, Premiums and Other Receivables	17				9				17	9
Premium Receivable, Credit Loss Expense (Reversal)									9
Premium Receivable, Allowance for Credit Loss, Writeoff									(2)
Premium Receivable, Allowance for Credit Loss, Recovery									0
Financing Receivable, Past Due	0								0
Financing Receivable, 90 Days or More Past Due, Still Accruing	0								0
Reinsurance Recoverable, Allowance for Credit Loss	0								0
satisfaction and discharge of segregated account deferred amounts																	$ 3,000
satisfaction and discharge of segregated account deferred interest amounts																	857
Loss and Loss Expense Benefit on Settled Deferred Amounts																	$ 288
Net Incurred RMBS Subrogation Recoveries									(23)	$ 42	62
Reinsurance Recoverable Credit Exposure	1								1
Accounting Standards Update 2016-13 [Member]
Insurance [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption	(4)								(4)
Accounting Standards Update 2016-13 [Member] | Premiums Receivable [Member]
Insurance [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption													$ (3)
Surveillance Category One [Member]
Insurance [Line Items]
Premiums Receivable, Gross	302								302
IA [Member]
Insurance [Line Items]
Premiums Receivable, Gross	40								40
II [Member]
Insurance [Line Items]
Premiums Receivable, Gross	3								3
III [Member]
Insurance [Line Items]
Premiums Receivable, Gross	27								27
IV [Member]
Insurance [Line Items]
Premiums Receivable, Gross	15								$ 15
Maximum [Member] | Furniture and Fixtures [Member]
Insurance [Line Items]
Finite-Lived Intangible Asset, Useful Life									5 years
Loss Reserves [Member]
Insurance [Line Items]
Weighted average risk-free rate used to discount loss reserves									1.10%	2.10%
United Kingdom, Pounds
Insurance [Line Items]
Financial Guarantee Insurance Contracts, Premium Receivable	117				129				$ 117	$ 129	$ 131	£ 86		£ 97		£ 103
Commonwealth of Puerto Rico [Member]
Insurance [Line Items]
Domestic Public Finance Losses									256
Public Finance [Member]
Insurance [Line Items]
Net par amount of financial guarantees	15,497				17,653				15,497	17,653
Premiums Receivable, Gross	185								185
Public Finance [Member] | Surveillance Category One [Member]
Insurance [Line Items]
Premiums Receivable, Gross	157								157
Public Finance [Member] | IA [Member]
Insurance [Line Items]
Premiums Receivable, Gross	27								27
Public Finance [Member] | II [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0								0
Public Finance [Member] | III [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0								0
Public Finance [Member] | IV [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0								0
Structured Finance [Member]
Insurance [Line Items]
Net par amount of financial guarantees	6,337				7,508				6,337	7,508
Premiums Receivable, Gross	59								59
Structured Finance [Member] | Surveillance Category One [Member]
Insurance [Line Items]
Premiums Receivable, Gross	27								27
Structured Finance [Member] | IA [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0								0
Structured Finance [Member] | II [Member]
Insurance [Line Items]
Premiums Receivable, Gross	3								3
Structured Finance [Member] | III [Member]
Insurance [Line Items]
Premiums Receivable, Gross	14								14
Structured Finance [Member] | IV [Member]
Insurance [Line Items]
Premiums Receivable, Gross	15								15
International Finance [Member]
Insurance [Line Items]
Net par amount of financial guarantees	12,054				12,857				12,054	12,857
Premiums Receivable, Gross	144								144
International Finance [Member] | Surveillance Category One [Member]
Insurance [Line Items]
Premiums Receivable, Gross	118								118
International Finance [Member] | IA [Member]
Insurance [Line Items]
Premiums Receivable, Gross	13								13
International Finance [Member] | II [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0								0
International Finance [Member] | III [Member]
Insurance [Line Items]
Premiums Receivable, Gross	13								13
International Finance [Member] | IV [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0								0
Housing Revenue [Member] | Public Finance [Member]
Insurance [Line Items]
Net par amount of financial guarantees	5,855				5,991				5,855	5,991
Premiums Receivable, Gross	168								168
Housing Revenue [Member] | Public Finance [Member] | Surveillance Category One [Member]
Insurance [Line Items]
Premiums Receivable, Gross	155								155
Housing Revenue [Member] | Public Finance [Member] | IA [Member]
Insurance [Line Items]
Premiums Receivable, Gross	13								13
Housing Revenue [Member] | Public Finance [Member] | II [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0								0
Housing Revenue [Member] | Public Finance [Member] | III [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0								0
Housing Revenue [Member] | Public Finance [Member] | IV [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0								0
Housing Revenue [Member] | Military Housing [Member] | Public Finance [Member]
Insurance [Line Items]
Net par amount of financial guarantees	5,575				5,654				5,575	5,654
Other Public Finance Sectors [Member] | Public Finance [Member]
Insurance [Line Items]
Premiums Receivable, Gross	17								17
Other Public Finance Sectors [Member] | Public Finance [Member] | Surveillance Category One [Member]
Insurance [Line Items]
Premiums Receivable, Gross	2								2
Other Public Finance Sectors [Member] | Public Finance [Member] | IA [Member]
Insurance [Line Items]
Premiums Receivable, Gross	15								15
Other Public Finance Sectors [Member] | Public Finance [Member] | II [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0								0
Other Public Finance Sectors [Member] | Public Finance [Member] | III [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0								0
Other Public Finance Sectors [Member] | Public Finance [Member] | IV [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0								0
Mortgage-backed and Home Equity [Member] | Structured Finance [Member]
Insurance [Line Items]
Net par amount of financial guarantees	3,635				4,423				3,635	4,423
Premiums Receivable, Gross	22								22
Mortgage-backed and Home Equity [Member] | Structured Finance [Member] | Surveillance Category One [Member]
Insurance [Line Items]
Premiums Receivable, Gross	3								3
Mortgage-backed and Home Equity [Member] | Structured Finance [Member] | IA [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0								0
Mortgage-backed and Home Equity [Member] | Structured Finance [Member] | II [Member]
Insurance [Line Items]
Premiums Receivable, Gross	1								1
Mortgage-backed and Home Equity [Member] | Structured Finance [Member] | III [Member]
Insurance [Line Items]
Premiums Receivable, Gross	3								3
Mortgage-backed and Home Equity [Member] | Structured Finance [Member] | IV [Member]
Insurance [Line Items]
Premiums Receivable, Gross	15								15
Structured Insurance [Member] | Structured Finance [Member]
Insurance [Line Items]
Net par amount of financial guarantees	311				395				311	395
Premiums Receivable, Gross	14								14
Structured Insurance [Member] | Structured Finance [Member] | Surveillance Category One [Member]
Insurance [Line Items]
Premiums Receivable, Gross	14								14
Structured Insurance [Member] | Structured Finance [Member] | IA [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0								0
Structured Insurance [Member] | Structured Finance [Member] | II [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0								0
Structured Insurance [Member] | Structured Finance [Member] | III [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0								0
Structured Insurance [Member] | Structured Finance [Member] | IV [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0								0
Student Loan [Member] | Structured Finance [Member]
Insurance [Line Items]
Net par amount of financial guarantees	626				769				626	769
Premiums Receivable, Gross	16								16
Student Loan [Member] | Structured Finance [Member] | Surveillance Category One [Member]
Insurance [Line Items]
Premiums Receivable, Gross	3								3
Student Loan [Member] | Structured Finance [Member] | IA [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0								0
Student Loan [Member] | Structured Finance [Member] | II [Member]
Insurance [Line Items]
Premiums Receivable, Gross	2								2
Student Loan [Member] | Structured Finance [Member] | III [Member]
Insurance [Line Items]
Premiums Receivable, Gross	11								11
Student Loan [Member] | Structured Finance [Member] | IV [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0								0
Other Structured Finance [Member] | Structured Finance [Member]
Insurance [Line Items]
Premiums Receivable, Gross	7								7
Other Structured Finance [Member] | Structured Finance [Member] | Surveillance Category One [Member]
Insurance [Line Items]
Premiums Receivable, Gross	7								7
Other Structured Finance [Member] | Structured Finance [Member] | IA [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0								0
Other Structured Finance [Member] | Structured Finance [Member] | II [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0								0
Other Structured Finance [Member] | Structured Finance [Member] | III [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0								0
Other Structured Finance [Member] | Structured Finance [Member] | IV [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0								0
Sovereign/Sub-sovereign [Member] | International Finance [Member]
Insurance [Line Items]
Net par amount of financial guarantees	5,270				5,264				5,270	5,264
Premiums Receivable, Gross	108								108
Sovereign/Sub-sovereign [Member] | International Finance [Member] | Surveillance Category One [Member]
Insurance [Line Items]
Premiums Receivable, Gross	82								82
Sovereign/Sub-sovereign [Member] | International Finance [Member] | IA [Member]
Insurance [Line Items]
Premiums Receivable, Gross	13								13
Sovereign/Sub-sovereign [Member] | International Finance [Member] | II [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0								0
Sovereign/Sub-sovereign [Member] | International Finance [Member] | III [Member]
Insurance [Line Items]
Premiums Receivable, Gross	13								13
Sovereign/Sub-sovereign [Member] | International Finance [Member] | IV [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0								0
Investor-owned and Public Utilities [Member] | International Finance [Member]
Insurance [Line Items]
Net par amount of financial guarantees	3,899				$ 4,436				3,899	$ 4,436
Premiums Receivable, Gross	31								31
Investor-owned and Public Utilities [Member] | International Finance [Member] | Surveillance Category One [Member]
Insurance [Line Items]
Premiums Receivable, Gross	31								31
Investor-owned and Public Utilities [Member] | International Finance [Member] | IA [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0								0
Investor-owned and Public Utilities [Member] | International Finance [Member] | II [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0								0
Investor-owned and Public Utilities [Member] | International Finance [Member] | III [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0								0
Investor-owned and Public Utilities [Member] | International Finance [Member] | IV [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0								0
Other International [Member] | International Finance [Member]
Insurance [Line Items]
Premiums Receivable, Gross	5								5
Other International [Member] | International Finance [Member] | Surveillance Category One [Member]
Insurance [Line Items]
Premiums Receivable, Gross	5								5
Other International [Member] | International Finance [Member] | IA [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0								0
Other International [Member] | International Finance [Member] | II [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0								0
Other International [Member] | International Finance [Member] | III [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0								0
Other International [Member] | International Finance [Member] | IV [Member]
Insurance [Line Items]
Premiums Receivable, Gross	$ 0								$ 0