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Commitments and Contingencies - Additional Information (Detail) $ in Millions | 12 Months Ended
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Dec. 31, 2018 USD ($) EquityUnit | Dec. 31, 2017 USD ($) | Dec. 31, 2016 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 | $ 4.6 | $ 4.7 | $ 4.5
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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- Definition Rental expense for the reporting period incurred under operating leases, including minimum and any contingent rent expense, net of related sublease income. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=77902758&loc=d3e41499-112717 + Details
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