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Variable Interest Entities - Additional Information (Detail) | 1 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Jul. 31, 2015 USD ($) | Dec. 31, 2018 USD ($) Entity | Dec. 31, 2017 USD ($) Entity | Dec. 31, 2016 USD ($) | Feb. 12, 2018 USD ($)
Variable Interest Entities [Line Items] |  |  |  |  |
satisfaction and discharge of segregated account deferred amounts |  |  |  |  | $ 3,000,158,000
satisfaction and discharge of segregated account deferred interest amounts |  |  |  |  | 856,834,000
Variable Interest Entity Change in Fair Value of Assets and Liabilities |  | $ 2,782,000 | $ 19,670,000 | $ (14,093,000) |
Available-for-sale Securities |  | 3,546,006,000 |  |  |
variable interest entities deconsolidation gain loss |  | 1,824,000 | 0 | 0 |
Variable Interest Entity, Measure of Activity, Income or Loss before Tax |  | 3,436,000 | 19,670,000 | (14,093,000) |
Secured Debt |  | 0 | 73,993,000 |  |
Fair value of special purpose entities |  | 4,516,000 | 5,979,000 |  |
Consolidated VIE assets |  | 7,093,309,000 | 14,500,507,000 |  |
Consolidated VIE liabilities |  | 6,981,244,000 | 14,366,434,000 |  |
Number of RMBS Sold | 17 |  |  |  |
Cash Received From Delaware Trust | $ 146,000,000 |  |  |  |
Securities Issued by Delaware Trust | $ 146,000,000 |  |  |  |
Equity Method Investments |  | 40,168,000 | 34,941,000 |  |
Loans, Unpaid principal balance |  | 19,191,000 | 20,184,000 |  |
Other Investments |  | 391,217,000 | 431,630,000 |  |
Deconsolidated VIE Assets |  | 6,619,857,000 |  |  |
Deconsolidated VIE Liabilities |  | 6,599,925,000 |  |  |
Long-term debt |  | $ 2,928,929,000 | 991,696,000 |  |
Asset-Backed Securities and Utility Obligations [Member] |  |  |  |  |
Variable Interest Entities [Line Items] |  |  |  |  |
Weighted average life |  | 2 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 |  | $ 393,010,000 | 420,600,000 |  |
Variable Interest Entity, Primary Beneficiary [Member] |  |  |  |  |
Variable Interest Entities [Line Items] |  |  |  |  |
Available-for-sale Securities |  | 2,737,286,000 | 2,914,145,000 |  |
Secured Debt |  |  |  | $ 0 |
Long-term debt |  | 5,268,596,000 | 12,160,544,000 |  |
Variable Interest Entities [Member] |  |  |  |  |
Variable Interest Entities [Line Items] |  |  |  |  |
Other Comprehensive Income (Loss), Financial Liability, Fair Value Option, Reclassification Adjustment from AOCI for Derecognition, before Tax |  | (1,170,000) |  |  |
Available-for-sale Securities |  | 2,737,286,000 | 2,914,145,000 |  |
Loans, Estimated fair value |  | 4,287,664,000 | 11,529,384,000 |  |
Loans, Unpaid principal balance |  | 3,402,413,000 | 8,168,651,000 |  |
Long-term Debt, Fair Value |  | 5,268,596,000 | 12,160,544,000 |  |
Long-term debt, Unpaid principal balance |  | $ 4,552,643,000 | $ 9,387,884,000 |  |
Consolidated Entities [Member] |  |  |  |  |
Variable Interest Entities [Line Items] |  |  |  |  |
Number of DeConsolidated Variable Interest Entities |  | 4 | 1 | 1 |
Number of consolidated Variable Interest Entities | Entity |  | 7 | 11 |  |
Ambac UK [Member] |  |  |  |  |
Variable Interest Entities [Line Items] |  |  |  |  |
Number of consolidated Variable Interest Entities | Entity |  | 7 | 8 |  |
Consolidated VIE assets |  | $ 7,093,309,000 | $ 14,160,152,000 |  |
Consolidated VIE liabilities |  | $ 6,981,244,000 | $ 14,026,704,000 |  |
Ambac Assurance [Member] |  |  |  |  |
Variable Interest Entities [Line Items] |  |  |  |  |
Number of consolidated Variable Interest Entities | Entity |  | 0 | 3 |  |
Secured Notes Received from Ambac LSNI |  |  |  |  | 643,583,000
Carry Value Secured Notes from LSNI |  | $ 584,622,000 |  |  |
Unsettled Amounts Secured Notes from LSNI |  | 19,405,000 |  |  |
AFG [Member] |  |  |  |  |
Variable Interest Entities [Line Items] |  |  |  |  |
Secured Notes Received from Ambac LSNI |  |  |  |  | $ 124,881,000
Carry Value Secured Notes from LSNI |  | 71,851,000 |  |  |
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] |  |  |  |  |
Variable Interest Entities [Line Items] |  |  |  |  |
Available-for-sale Securities |  | 2,737,286,000 | $ 2,914,145,000 |  |
Variable Interest Entity Purchased Debt [Member] |  |  |  |  |
Variable Interest Entities [Line Items] |  |  |  |  |
Other Investments |  | $ 0 | $ 35,000,000 |  |

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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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=109237563&loc=d3e33749-111570 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 280 -SubTopic 10 -Section 50 -Paragraph 25 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=115929826&loc=d3e8813-108599 Reference 3: 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 + 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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -URI http://asc.fasb.org/subtopic&trid=2196772 + Details
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