Basis of Presentation - Narrative (Details)		1 Months Ended	3 Months Ended			9 Months Ended		12 Months Ended
	Oct. 15, 2019 USD ($)	Mar. 31, 2019	Sep. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Dec. 31, 2018 partnership	Dec. 31, 2019 USD ($)	Mar. 01, 2019 USD ($)	May 31, 2018
Variable Interest Entity [Line Items]
Consolidation of multifamily loans held in securitization trusts						$ (3,849,779,000)	$ 1,481,554,000
Consolidation of multifamily ABS						$ (3,706,789,000)	1,408,002,000
Deconsolidation, realized market valuation gain (loss)				$ (72,000,000)
Number of partnerships consolidated | partnership								2
VIE, ownership interest rate						80.00%
Goodwill impairment expense			$ 0	89,000,000	$ 0	$ 88,675,000	0
Intangible assets impairment expense				0
Contingent consideration			0			0			$ 28,484,000
Acquisition related costs			135,000		$ 236,000	581,000	$ 547,000
Multifamily loans
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
Variable Interest Entity [Line Items]
Minority interest, percentage											20.00%
Percent acquired		80.00%
Intangible assets										$ 25,000,000
Contingent consideration			$ 14,000,000			$ 14,000,000				$ 25,000,000
Cash payment for contingent consideration liability				11,000,000
Restricted stock awards				$ 3,000,000
CoreVest
Variable Interest Entity [Line Items]
Intangible assets	$ 57,000,000
Restricted stock awards	$ 10,000,000
5 Arches LLC and CoreVest
Variable Interest Entity [Line Items]
Goodwill									89,000,000
Goodwill deductible for tax purposes									$ 75,000,000