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Special Purpose Entities, Including Variable Interest Entities - Additional Information (Detail) | 1 Months Ended | 12 Months Ended
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Jul. 31, 2015 USD ($) | Dec. 31, 2017 USD ($) Entity | Dec. 31, 2016 USD ($) Entity | Dec. 31, 2015 USD ($)
Variable Interest Entities [Line Items] |  |  |  |
Secured Debt |  | $ 73,993,000 | $ 102,403,000 |
Consolidated VIE assets |  | 14,500,507,000 | 13,367,834,000 |
Consolidated VIE liabilities |  | 14,366,434,000 | 13,235,425,000 |
Number of RMBS Sold | 17 |  |  |
Cash Received From Delaware Trust | $ 146,000,000 |  |  |
Par Value of Securities Placed in Trust |  | 293,409,000 |  |
Fair Value of Securities Placed in Trust |  | 346,212,000 |  |
Securities Issued by Delaware Trust | $ 146,000,000 |  |  |
Equity Method Investments |  | 34,941,000 | 30,003,000 |
Loans, Unpaid principal balance |  | 20,184,000 | 4,873,000 |
Successor [Member] |  |  |  |
Variable Interest Entities [Line Items] |  |  |  |
Variable Interest Entity Change in Fair Value of Assets and Liabilities |  | 19,670,000 | (14,093,000) | $ 30,997,000
Available-for-sale Securities |  | 5,309,161,000 | 6,049,908,000 |
variable interest entities deconsolidation gain loss |  | 0 | 0 | 572,000
Variable Interest Entity, Measure of Activity, Income or Loss before Tax |  | 19,670,000 | (14,093,000) | $ 31,569,000
Fair value of special purpose entities |  | 5,979,000 | 7,382,000 |
Other Investments |  | $ 431,630,000 | 450,307,000 |
Asset-Backed Securities and Utility Obligations [Member] |  |  |  |
Variable Interest Entities [Line Items] |  |  |  |
Weighted average life |  | 3 years 1 month 15 days |  |
Average rating of assets held by sponsored special purpose entities |  | BBB+ |  |
Sponsored Variable Interest Entities [Member] |  |  |  |
Variable Interest Entities [Line Items] |  |  |  |
Total principal amount of debt outstanding |  | $ 420,600,000 | 388,950,000 |
Variable Interest Entity, Primary Beneficiary [Member] |  |  |  |
Variable Interest Entities [Line Items] |  |  |  |
Secured Debt |  |  | 0 |
Variable Interest Entities [Member] | Successor [Member] |  |  |  |
Variable Interest Entities [Line Items] |  |  |  |
Available-for-sale Securities |  | 2,914,145,000 | 2,622,566,000 |
Loans, Estimated fair value |  | 11,529,384,000 | 10,658,963,000 |
Loans, Unpaid principal balance |  | 8,168,651,000 | 7,641,756,000 |
Long-term Debt, Fair Value |  | 12,160,544,000 | 11,155,936,000 |
Long-term debt, Unpaid principal balance |  | $ 9,387,884,000 | $ 8,854,530,000 |
Consolidated Entities [Member] |  |  |  |
Variable Interest Entities [Line Items] |  |  |  |
Number of DeConsolidated Variable Interest Entities |  | 1 | 1 | 2
Number of consolidated Variable Interest Entities | Entity |  | 11 | 12 |
Ambac UK [Member] |  |  |  |
Variable Interest Entities [Line Items] |  |  |  |
Number of consolidated Variable Interest Entities | Entity |  | 8 | 8 |
Consolidated VIE assets |  | $ 14,160,152,000 | $ 12,950,009,000 |
Consolidated VIE liabilities |  | $ 14,026,704,000 | $ 12,833,466,000 |
Ambac Assurance [Member] |  |  |  |
Variable Interest Entities [Line Items] |  |  |  |
Number of consolidated Variable Interest Entities | Entity |  | 3 | 4 |
London Interbank Offered Rate (LIBOR) [Member] |  |  |  |
Variable Interest Entities [Line Items] |  |  |  |
Interest Rate on Securities Issued by Delaware Trust | 2.80% |  |  |
Corporate Debt Securities [Member] | Successor [Member] |  |  |  |
Variable Interest Entities [Line Items] |  |  |  |
Available-for-sale Securities |  | $ 2,914,145,000 | $ 2,622,566,000 |
Variable Interest Entity Purchased Debt [Member] |  |  |  |
Variable Interest Entities [Line Items] |  |  |  |
Other Investments |  | $ 35,000,000 |  |

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- Definition The total principal amount of outstanding debt issued by Ambac's sponsored medium term note special purpose entities. + References No definition available. + Details
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- Definition Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26610-111562 + Details
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- Definition This item represents the carrying amount on the entity's balance sheet of its investment in common stock of an equity method investee. This is not an indicator of the fair value of the investment, rather it is the initial cost adjusted for the entity's share of earnings and losses of the investee, adjusted for any distributions (dividends) and other than temporary impairment (OTTI) losses recognized. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=66022249&loc=d3e33749-111570 Reference 2: http://www.xbrl.org/2003/role/presentationRef -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 + Details
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- Definition Fair value portion of loan receivable, including, but not limited to, mortgage loans held for investment, finance receivables held for investment, policy loans on insurance contracts. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -URI http://asc.fasb.org/subtopic&trid=2196772 + Details
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- Definition The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. + References No definition available. + Details
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- Definition Carrying value as of the balance sheet date, including the current and noncurrent portions, of collateralized debt obligations (with maturities initially due after one year or beyond the operating cycle, if longer). Such obligations include mortgage loans, chattel loans, and any other borrowings secured by assets of the borrower. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(16)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(22)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(a)(16)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 + Details
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- Definition Description of the amount of pre-tax income (loss) derived by the reporting entity from the Variable Interest Entity (VIE). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 50 -Paragraph 6 -Subparagraph d -URI http://asc.fasb.org/extlink&oid=77990344&loc=d3e5747-111685 + Details
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- Definition The carrying amount of the consolidated Variable Interest Entity's assets included in the reporting entity's statement of financial position. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (bb) -URI http://asc.fasb.org/extlink&oid=77990344&loc=d3e5710-111685 + Details
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- Definition The carrying amount of the consolidated Variable Interest Entity's liabilities included in the reporting entity's statement of financial position. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (bb) -URI http://asc.fasb.org/extlink&oid=77990344&loc=d3e5710-111685 + Details
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