Financial Guarantee Insurance Contracts - Additional Information (Detail) £ in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2019 GBP (£)	Feb. 12, 2019	Dec. 31, 2018 GBP (£)	Dec. 31, 2017 GBP (£)	Dec. 31, 2016 USD ($)
Insurance [Line Items]
Net par amount of financial guarantees	$ 38,018				$ 46,927				$ 38,018	$ 46,927
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	1,536				$ 1,633				1,536	$ 1,633	$ 1,645					$ 1,978
Stockholders Equity After Possible Increase in Domestic Public Finance Loss Reserves	536								536
Possible Increase in Domestic Public Finance Loss Reserves	$ 1,000								$ 1,000
Estimated Future Premium Payments Weighted Average Discounted Rate	2.40%				2.70%				2.40%	2.70%		2.40%		2.70%
Financial Guarantee Insurance Contracts, Premium Receivable	$ 416				$ 495				$ 416	$ 495	586					661
Amount Of Insured Par Outstanding Ceded To Reinsurer	2,746								$ 2,746
Ceded Principal Outstanding Major Reinsurer Percentage									6.30%
Financial Guarantee Gross Outstanding Principal Assumed From Third Parties	219				219				$ 219	219
Deferred Ceded Premiums And Reinsurance Recoverables	109								109
Reinsurance Payable	29				33				$ 29	$ 33
Weighted average period of future premiums									8 years 6 months	8 years 8 months 12 days
Uncollectable premium receivables	9				7				$ 9	$ 7
Accelerated premium revenue for retired obligations									10	32	64
Reinsurance recoveries of losses included in losses and loss expenses									(7)	(2)	20
Losses and loss expense reserves ceded to reinsurers	26				23				26	23	41					$ 31
Subrogation recoveries, net of reinsurance	(1,702)				(1,744)				(1,702)	(1,744)
Intangible amortization expense	15	$ 17	$ 226	$ 36	29	$ 26	$ 23	$ 29	295	107	151
Intangible Assets, Gross (Excluding Goodwill)	1,273				1,552				1,273	1,552
Finite-Lived Intangible Assets, Accumulated Amortization	847				833				847	833
Insurance intangible asset	427				719				427	719
Letters of Credit Outstanding, Amount	124								124
Financial Guarantee Outstanding Principal Ceded To Third Parties	5,890								5,890
Public Finance Puerto Rico Net Par Outstanding	1,123								1,123
Percent Cash & New COFINA Bonds Allocated Under POA													93.00%
Ambac Assurance Remaining Policy Obligations	101								101
Insurance Intangible Asset [Member]
Insurance [Line Items]
Intangible amortization expense									$ 295	$ 107	151
Loss Reserves [Member]
Insurance [Line Items]
Weighted average risk-free rate used to discount loss reserves									2.10%	2.80%
United Kingdom, Pounds
Insurance [Line Items]
Financial Guarantee Insurance Contracts, Premium Receivable	129				131				$ 129	$ 131	152	£ 97		£ 103	£ 112
Ambac UK [Member]
Insurance [Line Items]
Total Benefit Related to Ballantyne Settlement											$ 145
Commonwealth of Puerto Rico [Member]
Insurance [Line Items]
Domestic Public Finance Losses									250
Public Finance [Member]
Insurance [Line Items]
Net par amount of financial guarantees	17,653				23,442				17,653	23,442
Housing Revenue [Member] | Public Finance [Member]
Insurance [Line Items]
Net par amount of financial guarantees	5,991				6,159				5,991	6,159
Housing Revenue [Member] | Military Housing [Domain] | Public Finance [Member]
Insurance [Line Items]
Net par amount of financial guarantees	$ 5,654				$ 5,759				$ 5,654	$ 5,759