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Related-Party Transactions - Additional Information (Detail) - USD ($) | 12 Months Ended
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Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Related Party Transaction [Line Items] |  |  |
Operating lease cost | $ 24,372,000 |  |
Aggregate principal amount for employee notes receivable | 388,000 | $ 526,000 |
MMC [Member] |  |  |
Related Party Transaction [Line Items] |  |  |
Real estate brokerage commissions and financing fees from transactions with subsidiaries of Marcus & Millichap Company | 5,200,000 | 7,700,000 | $ 2,100,000
Commission expenses for transactions with subsidiaries of Marcus & Millichap Company | 3,000,000.0 | 4,600,000 | 1,200,000
Operating lease cost | $ 1,300,000 | 1,000,000.0 | 1,000,000.0
Lease expiration date | May 31, 2022 |  |
Accounts payable and other liabilities - related party | $ 88,000 | 101,000 |
MMC [Member] | Transition Services Agreement [Member] |  |  |
Related Party Transaction [Line Items] |  |  |
Selling, general and administrative expense | $ 127,000 | $ 197,000 | $ 210,000
George M. Marcus [Member] |  |  |
Related Party Transaction [Line Items] |  |  |
Beneficial ownership percentage | 40.00% |  |

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