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Derivative Instruments - Summary of Gross Principal Notional Outstanding for CDS Contracts (Detail) - Other Credit Derivatives [Member] - USD ($) $ in Thousands | Dec. 31, 2018 | Dec. 31, 2017
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Ambac Rating |  |
Notional outstanding | $ 295,342 | $ 325,890
AAA Rating [Member] |  |
Ambac Rating |  |
Notional outstanding | 0 | 0
AA Rating [Member] |  |
Ambac Rating |  |
Notional outstanding | 295,342 | 175,765
A Rating [Member] |  |
Ambac Rating |  |
Notional outstanding | 0 | 0
BBB Rating [Member] |  |
Ambac Rating |  |
Notional outstanding | 0 | 150,125
Below Investment Grade Rating [Member] |  |
Ambac Rating |  |
Notional outstanding | $ 0 | $ 0

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- Definition Aggregate notional amount specified by the derivative(s). Expressed as an absolute value. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 12 -Section 13 -Sentence Column B + Details
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- References No definition available. + Details
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