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