Financial Guarantee Insurance Contracts - Summary of Gross Premium Receivable Roll-Forward (Direct and Assumed Contracts) (Detail) $ in Thousands, € in Millions, £ in Millions	3 Months Ended						6 Months Ended
	Jun. 30, 2020 USD ($)	Jun. 30, 2020 EUR (€)	Jun. 30, 2020 GBP (£)	Jun. 30, 2019 USD ($)	Jun. 30, 2019 EUR (€)	Jun. 30, 2019 GBP (£)	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2020 USD ($)	Jan. 01, 2020 USD ($)	Dec. 31, 2019 USD ($)
Insurance [Line Items]
Allowance for Doubtful Accounts, Premiums and Other Receivables	$ 16,000						$ 16,000		$ 14,000		$ 9,000
Premiums Receivable, Gross	408,000						408,000
Financial Guarantee Insurance Contracts, Accelerated Premium Revenue, Amount	1,039			$ (6,000)			1,000	$ 6,000
Financial Guarantee Insurance Contracts, Premium Receivable [Roll Forward]
Beginning premium receivable							416,000	495,000
Premium receipts							22,000	26,000
Adjustments for changes in expected and contractual cash flows							9,000	(22,000)
Accretion of premium receivable discount							5,000	6,000
Uncollectable premiums							(4,000)	(1,000)
Other adjustments (including foreign exchange)							(8,000)	(11,000)
Ending premium receivable	392,000			442,000			392,000	442,000
Premium Receivable, Allowance for Credit Loss	16,285						16,285
Premium Receivable, Credit Loss Expense (Reversal)	2,000						7,000
Premium Receivable, Allowance for Credit Loss, Writeoff	0						0
Premium Receivable, Allowance for Credit Loss, Recovery	0						0
Financing Receivable, Past Due	2,000						2,000
Financing Receivable, 90 Days or More Past Due, Still Accruing	1,000						1,000
Reinsurance Recoverable, Allowance for Credit Loss	499						499
United Kingdom, Pounds
Financial Guarantee Insurance Contracts, Premium Receivable [Roll Forward]
Ending premium receivable	127,000		£ 103	137,000		£ 108	127,000	137,000
Euro Member Countries, Euro
Financial Guarantee Insurance Contracts, Premium Receivable [Roll Forward]
Ending premium receivable	22,000	€ 19		$ 28,000	€ 25		22,000	$ 28,000
Accounting Standards Update 2016-13 [Member]
Insurance [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption										$ (4,000)
Premiums Receivable [Member] | Accounting Standards Update 2016-13 [Member]
Insurance [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption										$ (3,000)
Surveillance Category Onea [Member]
Insurance [Line Items]
Premiums Receivable, Gross	39,000						39,000
Surveillance Category Two [Member]
Insurance [Line Items]
Premiums Receivable, Gross	4,000						4,000
Surveillance Category Three [Member]
Insurance [Line Items]
Premiums Receivable, Gross	29,000						29,000
Surveillance Category Four [Member]
Insurance [Line Items]
Premiums Receivable, Gross	17,000						17,000
Surveillance Category One [Member]
Insurance [Line Items]
Premiums Receivable, Gross	320,000						320,000
Structured Finance [Member]
Insurance [Line Items]
Premiums Receivable, Gross	65,000						65,000
Structured Finance [Member] | Surveillance Category Onea [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0						0
Structured Finance [Member] | Surveillance Category Two [Member]
Insurance [Line Items]
Premiums Receivable, Gross	4,000						4,000
Structured Finance [Member] | Surveillance Category Three [Member]
Insurance [Line Items]
Premiums Receivable, Gross	15,000						15,000
Structured Finance [Member] | Surveillance Category Four [Member]
Insurance [Line Items]
Premiums Receivable, Gross	17,000						17,000
Structured Finance [Member] | Surveillance Category One [Member]
Insurance [Line Items]
Premiums Receivable, Gross	29,000						29,000
International Finance [Member]
Insurance [Line Items]
Premiums Receivable, Gross	154,000						154,000
International Finance [Member] | Surveillance Category Onea [Member]
Insurance [Line Items]
Premiums Receivable, Gross	12,000						12,000
International Finance [Member] | Surveillance Category Two [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0						0
International Finance [Member] | Surveillance Category Three [Member]
Insurance [Line Items]
Premiums Receivable, Gross	14,000						14,000
International Finance [Member] | Surveillance Category Four [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0						0
International Finance [Member] | Surveillance Category One [Member]
Insurance [Line Items]
Premiums Receivable, Gross	128,000						128,000
Public Finance [Member]
Insurance [Line Items]
Premiums Receivable, Gross	189,000						189,000
Public Finance [Member] | Surveillance Category Onea [Member]
Insurance [Line Items]
Premiums Receivable, Gross	26,000						26,000
Public Finance [Member] | Surveillance Category Two [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0						0
Public Finance [Member] | Surveillance Category Three [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0						0
Public Finance [Member] | Surveillance Category Four [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0						0
Public Finance [Member] | Surveillance Category One [Member]
Insurance [Line Items]
Premiums Receivable, Gross	162,000						162,000
Other Public Finance Sectors [Member] | Public Finance [Member]
Insurance [Line Items]
Premiums Receivable, Gross	16,000						16,000
Other Public Finance Sectors [Member] | Public Finance [Member] | Surveillance Category Onea [Member]
Insurance [Line Items]
Premiums Receivable, Gross	13,000						13,000
Other Public Finance Sectors [Member] | Public Finance [Member] | Surveillance Category Two [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0						0
Other Public Finance Sectors [Member] | Public Finance [Member] | Surveillance Category Three [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0						0
Other Public Finance Sectors [Member] | Public Finance [Member] | Surveillance Category Four [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0						0
Other Public Finance Sectors [Member] | Public Finance [Member] | Surveillance Category One [Member]
Insurance [Line Items]
Premiums Receivable, Gross	2,000						2,000
Other International [Member] | International Finance [Member]
Insurance [Line Items]
Premiums Receivable, Gross	13,000						13,000
Other International [Member] | International Finance [Member] | Surveillance Category Onea [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0						0
Other International [Member] | International Finance [Member] | Surveillance Category Two [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0						0
Other International [Member] | International Finance [Member] | Surveillance Category Three [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0						0
Other International [Member] | International Finance [Member] | Surveillance Category Four [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0						0
Other International [Member] | International Finance [Member] | Surveillance Category One [Member]
Insurance [Line Items]
Premiums Receivable, Gross	13,000						13,000
Mortgage Backed And Home Equity [Member] | Structured Finance [Member]
Insurance [Line Items]
Premiums Receivable, Gross	25,000						25,000
Mortgage Backed And Home Equity [Member] | Structured Finance [Member] | Surveillance Category Onea [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0						0
Mortgage Backed And Home Equity [Member] | Structured Finance [Member] | Surveillance Category Two [Member]
Insurance [Line Items]
Premiums Receivable, Gross	1,000						1,000
Mortgage Backed And Home Equity [Member] | Structured Finance [Member] | Surveillance Category Three [Member]
Insurance [Line Items]
Premiums Receivable, Gross	3,000						3,000
Mortgage Backed And Home Equity [Member] | Structured Finance [Member] | Surveillance Category Four [Member]
Insurance [Line Items]
Premiums Receivable, Gross	17,000						17,000
Mortgage Backed And Home Equity [Member] | Structured Finance [Member] | Surveillance Category One [Member]
Insurance [Line Items]
Premiums Receivable, Gross	3,000						3,000
Structured Insurance [Domain] | Structured Finance [Member]
Insurance [Line Items]
Premiums Receivable, Gross	17,000						17,000
Structured Insurance [Domain] | Structured Finance [Member] | Surveillance Category Onea [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0						0
Structured Insurance [Domain] | Structured Finance [Member] | Surveillance Category Two [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0						0
Structured Insurance [Domain] | Structured Finance [Member] | Surveillance Category Three [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0						0
Structured Insurance [Domain] | Structured Finance [Member] | Surveillance Category Four [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0						0
Structured Insurance [Domain] | Structured Finance [Member] | Surveillance Category One [Member]
Insurance [Line Items]
Premiums Receivable, Gross	17,000						17,000
Student Loans [Member] | Structured Finance [Member]
Insurance [Line Items]
Premiums Receivable, Gross	18,000						18,000
Student Loans [Member] | Structured Finance [Member] | Surveillance Category Onea [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0						0
Student Loans [Member] | Structured Finance [Member] | Surveillance Category Two [Member]
Insurance [Line Items]
Premiums Receivable, Gross	2,000						2,000
Student Loans [Member] | Structured Finance [Member] | Surveillance Category Three [Member]
Insurance [Line Items]
Premiums Receivable, Gross	12,000						12,000
Student Loans [Member] | Structured Finance [Member] | Surveillance Category Four [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0						0
Student Loans [Member] | Structured Finance [Member] | Surveillance Category One [Member]
Insurance [Line Items]
Premiums Receivable, Gross	3,000						3,000
Other Structured Finance [Member] | Structured Finance [Member]
Insurance [Line Items]
Premiums Receivable, Gross	6,000						6,000
Other Structured Finance [Member] | Structured Finance [Member] | Surveillance Category Onea [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0						0
Other Structured Finance [Member] | Structured Finance [Member] | Surveillance Category Two [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0						0
Other Structured Finance [Member] | Structured Finance [Member] | Surveillance Category Three [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0						0
Other Structured Finance [Member] | Structured Finance [Member] | Surveillance Category Four [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0						0
Other Structured Finance [Member] | Structured Finance [Member] | Surveillance Category One [Member]
Insurance [Line Items]
Premiums Receivable, Gross	6,000						6,000
Sovereign And Sub Sovereign [Member] | International Finance [Member]
Insurance [Line Items]
Premiums Receivable, Gross	113,000						113,000
Sovereign And Sub Sovereign [Member] | International Finance [Member] | Surveillance Category Onea [Member]
Insurance [Line Items]
Premiums Receivable, Gross	12,000						12,000
Sovereign And Sub Sovereign [Member] | International Finance [Member] | Surveillance Category Two [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0						0
Sovereign And Sub Sovereign [Member] | International Finance [Member] | Surveillance Category Three [Member]
Insurance [Line Items]
Premiums Receivable, Gross	14,000						14,000
Sovereign And Sub Sovereign [Member] | International Finance [Member] | Surveillance Category Four [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0						0
Sovereign And Sub Sovereign [Member] | International Finance [Member] | Surveillance Category One [Member]
Insurance [Line Items]
Premiums Receivable, Gross	87,000						87,000
Investor Owned And Public Utilities [Member] | International Finance [Member]
Insurance [Line Items]
Premiums Receivable, Gross	28,000						28,000
Investor Owned And Public Utilities [Member] | International Finance [Member] | Surveillance Category Onea [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0						0
Investor Owned And Public Utilities [Member] | International Finance [Member] | Surveillance Category Two [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0						0
Investor Owned And Public Utilities [Member] | International Finance [Member] | Surveillance Category Three [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0						0
Investor Owned And Public Utilities [Member] | International Finance [Member] | Surveillance Category Four [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0						0
Investor Owned And Public Utilities [Member] | International Finance [Member] | Surveillance Category One [Member]
Insurance [Line Items]
Premiums Receivable, Gross	28,000						28,000
Housing Revenue [Member] | Public Finance [Member]
Insurance [Line Items]
Premiums Receivable, Gross	173,000						173,000
Housing Revenue [Member] | Public Finance [Member] | Surveillance Category Onea [Member]
Insurance [Line Items]
Premiums Receivable, Gross	13,000						13,000
Housing Revenue [Member] | Public Finance [Member] | Surveillance Category Two [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0						0
Housing Revenue [Member] | Public Finance [Member] | Surveillance Category Three [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0						0
Housing Revenue [Member] | Public Finance [Member] | Surveillance Category Four [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0						0
Housing Revenue [Member] | Public Finance [Member] | Surveillance Category One [Member]
Insurance [Line Items]
Premiums Receivable, Gross	$ 160,000						$ 160,000