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Commitments and Contingencies - Additional Information (Detail) $ in Millions | 12 Months Ended
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Dec. 31, 2017 USD ($) EquityUnit | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($)
Contingencies And Commitments [Line Items] |  |  |
Number of private equity funds | EquityUnit | 2 |  |
Commitments towards private equity funds | $ 7.7 |  |
Operating leases expiration year | 2025 |  |
Rental expense | $ 4.5 | $ 4.2 | $ 4.9
Investment Commitments [Member] |  |  |
Contingencies And Commitments [Line Items] |  |  |
Aggregate investment committed in private equity funds | 7.9 |  |
Private Equity Funds [Member] |  |  |
Contingencies And Commitments [Line Items] |  |  |
Contribution to private equity funds | $ 8.4 |  |

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