Financial Guarantee Insurance Contracts - Additional Information (Detail) € in Millions, £ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2020 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2019 EUR (€)	Mar. 31, 2020 EUR (€)	Mar. 31, 2020 GBP (£)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 GBP (£)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Insurance [Line Items]
Reinsurance Recoverable, Allowance for Credit Loss						$ 0
Public Finance Puerto Rico Net Par Outstanding						1,105,000,000
Schedule Of Insured Financial Obligations With Credit Deterioration Reductions Of Gross Claim Liability Rmbs Subrogation						1,764,000,000		$ 1,727,000,000
Financial Guarantee Insurance Contracts, Premium Receivable		$ 487,000,000				$ 403,000,000		$ 416,000,000	$ 495,000,000
Estimated Future Premium Payments Weighted Average Discounted Rate			2.40%	2.20%	2.20%	2.20%	2.40%	2.40%
Reinsurance Payable						$ 28,000,000		$ 29,000,000
Weighted average period of future premiums	8 years 8 months 12 days		8 years 6 months
Uncollectable premium receivables						14,352,000
Accelerated premium revenue for retired obligations	$ 187,000	12,000,000
Reinsurance recoveries of losses included in losses and loss expenses	(10,000,000)	(5,000,000)
Losses and loss expense reserves ceded to reinsurers		26,000,000				35,000,000		26,000,000	23,000,000
Subrogation recoveries, net of reinsurance						(1,738,000,000)		(1,702,000,000)
Intangible amortization expense	$ 13,000,000	36,000,000
Intangible Assets, Gross (Excluding Goodwill)						1,261,000,000		1,273,000,000
Finite-Lived Intangible Assets, Accumulated Amortization						854,000,000		847,000,000
Insurance intangible asset						406,000,000		427,000,000
Possible Increase in Loss Reserves Related to Puerto Rico						1,220,000,000
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest		$ 1,666,000,000				1,062,000,000		1,536,000,000	$ 1,633,000,000
Stockholders Equity After Possible Increase in Domestic Public Finance Loss Reserves						(158,000,000)
Loss Reserves [Member]
Insurance [Line Items]
Weighted average risk-free rate used to discount loss reserves	0.90%		2.10%
United Kingdom, Pounds
Insurance [Line Items]
Financial Guarantee Insurance Contracts, Premium Receivable					£ 103	128,000,000	£ 109	142,000,000
Euro Member Countries, Euro
Insurance [Line Items]
Financial Guarantee Insurance Contracts, Premium Receivable			€ 27	€ 22		24,000,000		$ 30,000,000
Commonwealth of Puerto Rico [Member]
Insurance [Line Items]
Domestic Public Finance Losses	$ 178,000,000
Long-term Debt [Member]
Insurance [Line Items]
Gain Contingency, Unrecorded Amount						1,600,000,000
Long-term Debt [Member]
Insurance [Line Items]
Gain Contingency, Unrecorded Amount						$ 1,400,000,000