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Investments - Investments in Variable Interest Entities (Parenthetical) (Detail) - USD ($) | Mar. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Variable Interest Entity [Line Items] |  |
Principal sum of loan due from related parties | $ 6,579,000 | $ 3,739,000
Aqua [Member] |  |
Variable Interest Entity [Line Items] |  |
Principal sum of loan due from related parties | 980,000 |
Subordinated Loan [Member] | Aqua [Member] |  |
Variable Interest Entity [Line Items] |  |
Maximum Exposure to Loss | $ 980,000 | $ 980,000

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