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Special Purpose Entities, Including Variable Interest Entities - Summary of Fair Value of Fixed Income Securities, by Asset-Type, Held by Consolidated Variable Interest Entities (Detail) - USD ($) $ in Thousands | Jun. 30, 2019 | Dec. 31, 2018
--- | --- | ---
Variable Interest Entities [Line Items] |  |
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744) | $ 3,322,599 | $ 3,451,149
Investments: |  |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 140,254 | 133,052
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 1,959 | 38,195
Variable Interest Entity, Primary Beneficiary, Aggregated Disclosure [Member] |  |
Investments: |  |
Loans Unpaid Principal Balance | 3,342,724 | 3,418,044
Long Term Debt Unpaid Principal Balance | 4,474,815 | $ 4,552,643
Municipal Bonds [Member] | Variable Interest Entity, Primary Beneficiary [Member] |  |
Variable Interest Entities [Line Items] |  |
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744) | 236,465 |
Investments: |  |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | $ 20,360 |

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