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Investments - Additional Information (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Investment [Line Items]
Other investments	$ 428,556	$ 391,217
Securities fair value	6,046	5,975
Ambac Assurance [Member]
Investment [Line Items]
Unsettled Amounts Secured Notes from LSNI	0	19,405
Ambac UK [Member]
Investment [Line Items]
Asset Backed Securities Fair Value Disclosure	144,530	144,672
Interest Rate Contract [Member]
Investment [Line Items]
Other investments	207,704	177,357
Reported Value Measurement [Member] | Interest Rate Contract [Member]
Investment [Line Items]
Other investments	$ 53,135	$ 27,154

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Corporate and Asset Backed Securities Fair Value Disclosure

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- Definition

Line items represent reporting 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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 12
-Sentence Column A

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- Definition

Amount of investments classified as other.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.1(f,g))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.12)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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