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Financial Guarantee Insurance Contracts - Summary of Gross Premium Receivable Roll-Forward (Direct and Assumed Contracts) (Detail) € in Thousands, £ in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($)	Dec. 31, 2018 EUR (€)	Dec. 31, 2018 GBP (£)	Dec. 31, 2017 USD ($)	Dec. 31, 2017 EUR (€)	Dec. 31, 2017 GBP (£)	Dec. 31, 2016 USD ($)
Insurance [Line Items]
Premiums Receivable, Allowance for Doubtful Accounts	$ 7,136			$ 9,331
Financial Guarantee Insurance Contracts, Premium Receivable [Roll Forward]
Beginning premium receivable	586,312			661,337			$ 831,575
Premium receipts	56,441			81,597			(77,038)
Adjustments for changes in expected and contractual cash flows	(41,762)			(30,334)			(78,528)
Accretion of premium receivable discount	14,668			16,162			18,637
Uncollectable premiums	2,167			(141)			6,054
Other adjustments (including foreign exchange)	(9,553)			20,885			(39,363)
Ending premium receivable	495,391			586,312			661,337
United Kingdom, Pounds
Financial Guarantee Insurance Contracts, Premium Receivable [Roll Forward]
Beginning premium receivable	151,852		£ 112,342	177,878		£ 144,393
Ending premium receivable	131,458		£ 103,088	151,852		£ 112,342	177,878
Euro Member Countries, Euro
Financial Guarantee Insurance Contracts, Premium Receivable [Roll Forward]
Beginning premium receivable	36,001	€ 29,976		34,866	€ 33,108
Ending premium receivable	$ 30,597	€ 26,708		$ 36,001	€ 29,976		$ 34,866
External Credit Rating, Non Investment Grade [Member]
Financial Guarantee Insurance Contracts, Premium Receivable [Roll Forward]
Financial Guarantee Insurance Contracts, Premium Receivable Percent	20.00%	20.00%	20.00%	22.00%	22.00%	22.00%

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Amount of increase (decrease) in the allowance for doubtful accounts.

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Amount of the premium receivable for financial guarantee insurance contracts reported in the statement of financial position. This element primarily would consist of those financial guarantee insurance contracts which premiums are to be collected over the contract period, but may, on occasion, include premiums which are to be collected in full at inception, but which straddle a financial reporting period.

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Amount of cash inflow from premiums received for premium receivable on financial guarantee insurance contracts.

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Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Amount of accretion of the discount for premiums receivable on financial guarantee insurance contracts.

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Amount of increase (decrease) in the premium receivable on financial guarantee insurance contracts from changes in the period of the contract.

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Amount of increase (decrease) in the premiums receivable on financial guarantee insurance contracts from adjustments classified as other.

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The carrying value as of the balance sheet date of the estimated valuation allowance to reduce gross premiums receivable to net realizable value.

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