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Financial Guarantees in Force - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Guarantor Obligations [Line Items]
Financial Guarantee Insurance Contracts, Accelerated Premium Revenue, Amount	$ 32,482	$ 64,494	$ 52,416
Gross par amount of financial guarantees outstanding	52,055,000	67,140,000
Net par amount of financial guarantees	46,927,000	62,716,000
Gross financial guarantees in force	87,543,000	108,550,000
Net financial guarantees in force	$ 77,972,000	101,223,000
Highest single insured risk of aggregate net par amount guaranteed	2.80%
California [Member]
Guarantor Obligations [Line Items]
Aggregate net par amounts in force	7.90%
New York [Member]
Guarantor Obligations [Line Items]
Aggregate net par amounts in force	5.50%
NEW JERSEY
Guarantor Obligations [Line Items]
Aggregate net par amounts in force	5.20%
No other state [Member] | Maximum [Member]
Guarantor Obligations [Line Items]
Aggregate net par amounts in force	5.00%
Public Finance [Member]
Guarantor Obligations [Line Items]
Net par amount of financial guarantees	$ 23,442,000	32,088,000
Public Finance [Member] | Housing Revenue [Member]
Guarantor Obligations [Line Items]
Net par amount of financial guarantees	6,159,000	6,312,000
Military Housing [Domain] | Public Finance [Member] | Housing Revenue [Member]
Guarantor Obligations [Line Items]
Net par amount of financial guarantees	$ 5,759,000	$ 5,829,000

X

- Definition

Financial guarantee insurance gross outstanding principal.

+ References

No definition available.

+ Details

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- Definition

Financial guarantee insurance gross outstanding principal and interest.

+ References

No definition available.

+ Details

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X

- Definition

Financial guarantee insurance outstanding principal and interest, net of reinsurance.

+ References

No definition available.

+ Details

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X

- Definition

Percentage of related member outstanding par, net of reinsurance, as compared to the Company's total outstanding financial guarantee par, net of reinsurance.

+ References

No definition available.

+ Details

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X

- Definition

Percentage of highest single insured risk of aggregate net par amount guaranteed

+ References

No definition available.

+ Details

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X

- Definition

This element represents the amount of premium revenue on financial guarantee insurance contracts recognized during the period that was accelerated for revenue recognition purposes.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 605
-Section 50
-Paragraph 4
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=6933581&loc=SL5749298-161291

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X

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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- Definition

Amount of participating insurance in force.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 50
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=6483535&loc=d3e16392-158451

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