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Investments - Other Investments - Additional Information (Detail) | Dec. 31, 2017 USD ($) Investment Venture | Dec. 31, 2016 USD ($)
--- | --- | ---
Schedule of Equity Method Investments [Line Items] |  |
Assets | $ 79,100,000 | $ 93,700,000
Non controlling interest certified low-income housing developments | Investment | 8 |
Other Investments [Member] |  |
Schedule of Equity Method Investments [Line Items] |  |
Assets | $ 3,800,000 | $ 3,700,000
Other Investments [Member] | Four Venture Capital Funds [Member] |  |
Schedule of Equity Method Investments [Line Items] |  |
Assets | $ 3,800,000 |
Non controlling interest in venture capital funds number | Venture | 4 |
Other Investments [Member] | Variable Interest Entity, Not Primary Beneficiary [Member] |  |
Schedule of Equity Method Investments [Line Items] |  |
Assets | $ 15,000,000 |
Liabilities | 0 |
Other Investments [Member] | Variable Interest Entity, Not Primary Beneficiary [Member] | Eight Certified Low Income Housing Developments [Member] |  |
Schedule of Equity Method Investments [Line Items] |  |
Carrying value of investments | $ 0 |

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