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Investments - Summary of Roll-Forward of Ambac's Cumulative Credit Losses on Debt Securities for Which Portion of Other-than-Temporary Impairment was Recognized in Other Comprehensive Income (Detail) - Successor [Member] - USD ($) $ in Thousands | 6 Months Ended |  |
--- | --- | --- | ---
Jun. 30, 2018 | Jun. 30, 2017 | Dec. 31, 2017 | Dec. 31, 2016
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward] |  |  |  |
Balance at beginning | $ 38,732 | $ 54,362 | $ 67,085 | $ 52,070
Additions for credit impairments recognized on: |  |  |  |
Securities not previously impaired | 226 | 307 |  |
Securities previously impaired | 98 | 1,985 |  |
Reductions for credit impairments previously recognized on: |  |  |  |
Securities that matured or were sold during the period | 28,677 | 0 |  |
Balance at end | $ 38,732 | $ 54,362 | $ 67,085 | $ 52,070

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- Definition Other than temporary impairments additions for credit impairments recognized. + References No definition available. + Details
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- Definition Other than temporary impairments reductions for credit impairments previously recognized. + References No definition available. + Details
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- Definition This item represents an increase to the cumulative amount of credit losses recognized in earnings for an other than temporary impairment (OTTI) of a debt security held for which a previous OTTI was recognized and the investor does not intend to sell the debt security and it is not more likely than not that the investor will be required to sell the debt security before recovery of its amortized cost basis. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 8B -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=82897076&loc=SL6284393-111563 + Details
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- Definition This item represents an increase to the cumulative amount of credit losses recognized in earnings for an other than temporary impairment (OTTI) of a debt security held for which no such other than temporary impairment (OTTI) was previously recognized. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 8B -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=82897076&loc=SL6284393-111563 + Details
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- Definition The cumulative amount of credit losses recognized in earnings related to debt securities held for which a portion of an other than temporary impairment (OTTI) was recognized in other comprehensive income (a component of shareholders' equity). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 8B -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=82897076&loc=SL6284393-111563 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 8B -Subparagraph (g) -URI http://asc.fasb.org/extlink&oid=82897076&loc=SL6284393-111563 + Details
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- Definition This item represents reductions in the cumulative amount of credit losses recognized in earnings for an other than temporary impairment (OTTI) of a debt security because of the sale of impaired securities during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 8B -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=82897076&loc=SL6284393-111563 + Details
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- Definition A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period. + References No definition available. + Details
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