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Basis of Presentation - Narrative (Details) |  |  | 1 Months Ended | 3 Months Ended | 12 Months Ended
--- | --- | --- | --- | --- | ---
Oct. 15, 2019 USD ($) | Mar. 01, 2019 USD ($) | Oct. 31, 2019 USD ($) | Mar. 31, 2019 | May 31, 2018 USD ($) | Sep. 30, 2020 | Mar. 31, 2020 USD ($) | Sep. 30, 2019 partnership | Mar. 31, 2019 USD ($) | Dec. 31, 2020 USD ($) | Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($) partnership
Variable Interest Entity [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Deconsolidation, realized market valuation gain (loss) |  |  |  |  |  |  | $ (72,000,000) |  |  |  |  |
Number of partnerships consolidated | partnership |  |  |  |  |  |  |  | 2 |  |  |  | 2
VIE, ownership interest rate |  |  |  |  |  | 80.00% |  |  |  |  |  |
Contingent consideration |  |  |  |  |  |  |  |  |  | $ 0 | $ 28,484,000 |
Contingent consideration expense |  |  |  |  |  |  |  |  |  | 200,000 | 3,218,000 | $ 0
Acquisition-related equity compensation expense |  |  |  |  |  |  |  |  |  | 4,848,000 | 1,010,000 | 0
Gain on re-measurement of 5 Arches investment |  |  |  |  |  |  |  |  |  | 0 | 2,441,000 | 0
Amortization of Intangible Assets |  |  |  |  |  |  |  |  |  | 16,000,000 | 9,000,000 |
Goodwill impairment expense |  |  |  |  |  |  | 89,000,000 |  |  | 88,675,000 | 0 | $ 0
Intangible asset impairment expense |  |  |  |  |  |  | 0 |  |  |  |  |
5 Arches, LLC |  |  |  |  |  |  |  |  |  |  |  |
Variable Interest Entity [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Payments to acquire equity method investment |  |  |  |  | $ 10,000,000 |  |  |  |  |  |  |
Option to purchase additional equity, term |  |  |  |  | 1 year |  |  |  |  |  |  |
Multifamily securities |  |  |  |  |  |  |  |  |  |  |  |
Variable Interest Entity [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Consolidation of multifamily loans held in securitization trusts |  |  |  |  |  |  | 3,860,000,000 |  |  |  |  |
Consolidation of multifamily ABS |  |  |  |  |  |  | 3,720,000,000 |  |  |  |  |
5 Arches, LLC |  |  |  |  |  |  |  |  |  |  |  |
Variable Interest Entity [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Minority interest, percentage |  |  |  |  | 20.00% |  |  |  |  |  |  |
Option to purchase additional equity, percent |  |  |  | 80.00% |  |  |  |  |  |  |  |
Cash |  | $ 12,575,000 |  |  |  |  |  |  |  |  |  |
Contingent consideration maximum amount |  | 27,000,000 |  |  | $ 29,000,000 |  |  |  |  |  |  |
Contingent consideration performance term |  |  |  |  | 2 years |  |  |  |  |  |  |
Contingent consideration |  | 24,621,000 |  |  |  |  |  |  |  | 15,000,000 |  |
Cash payment for contingent consideration |  |  |  |  |  |  | 11,000,000 |  |  |  |  |
Restricted stock awards |  |  |  |  |  |  | $ 3,000,000 |  |  |  |  |
Contingent consideration expense |  |  |  |  |  |  |  |  |  | 249,000 |  |
Gain on re-measurement of 5 Arches investment |  |  |  |  |  |  |  |  | $ 2,000,000 |  |  |
Intangible assets |  | 24,800,000 |  |  |  |  |  |  |  |  |  |
Goodwill |  | $ 28,747,000 |  |  |  |  |  |  |  |  |  |
CoreVest LLC |  |  |  |  |  |  |  |  |  |  |  |
Variable Interest Entity [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Cash | $ 482,311,000 |  | $ 482,000,000 |  |  |  |  |  |  |  |  |
Contingent consideration performance term | 2 years |  |  |  |  |  |  |  |  |  |  |
Contingent consideration | $ 0 |  |  |  |  |  |  |  |  |  |  |
Restricted stock awards | 10,000,000 |  | 10,000,000 |  |  |  |  |  |  |  |  |
Total consideration |  |  | 492,000,000 |  |  |  |  |  |  |  |  |
Acquisition-related equity compensation expense |  |  | $ 10,000,000 |  |  |  |  |  |  |  |  |
Intangible assets | 56,500,000 |  |  |  |  |  |  |  |  |  |  |
Goodwill | $ 59,928,000 |  |  |  |  |  |  |  |  |  |  |
5 Arches LLC And CoreVest |  |  |  |  |  |  |  |  |  |  |  |
Variable Interest Entity [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Contingent consideration expense |  |  |  |  |  |  |  |  |  |  | 2,000,000 |
Amortization of Intangible Assets |  |  |  |  |  |  |  |  |  | $ 24,435,000 | 8,511,000 |
Goodwill |  |  |  |  |  |  |  |  |  |  | 89,000,000 |
Goodwill deductible for tax purposes |  |  |  |  |  |  |  |  |  |  | $ 75,000,000 |

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