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Real Estate - Additional Information (Detail) $ in Millions | 6 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | ---
Jun. 30, 2018 USD ($) RenewalOption | Jun. 30, 2017 USD ($) | Dec. 31, 2017 USD ($) | Apr. 30, 2018 USD ($) ft² | Dec. 31, 2008 USD ($)
Real Estate Properties [Line Items] |  |  |  |  |
Net deferred gain on real estate sale | $ 8.4 |  |  |  | $ 39.1
Deferred gain recognized on real estate sale | $ 0.4 | $ 1.8 |  |  |
Lease expiration year | 2029 |  |  |  |
Percentage of management fee included in the base annual rent |  |  | 3.50% |  |
Lease security deposit |  |  | $ 4.7 |  |
Letter of credit |  |  | $ 3.0 |  |
Number of times to renew the lease contract | RenewalOption | 2 |  |  |  |
Lease extension period | 10 years |  |  |  |
Amended Lease Term |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |
Lease expiration date | Dec. 31, 2029 |  |  |  |
Percentage reduction in base rental |  |  | 8.00% |  |
Tenant improvement allowances |  |  | $ 13.1 |  |
Lease security deposit |  |  | $ 3.0 |  |
Commercial Lease Agreement |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |
Office space area leased | ft² |  |  |  | 44,718 |
Lease term |  |  |  | 130 months |
Commercial Lease Agreement | Letter of Credit |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |
Lease security deposit |  |  |  | $ 1.0 |

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