Principles of Consolidation - Additional Information (Details) $ in Thousands	3 Months Ended	9 Months Ended		12 Months Ended
	Mar. 31, 2020 USD ($)	Sep. 30, 2020 USD ($) entity	Sep. 30, 2019 USD ($)	Dec. 31, 2018 partnership	Dec. 31, 2019 USD ($)
Variable Interest Entity [Line Items]
Consolidation of multifamily loans held in securitization trusts		$ (3,849,779)	$ 1,481,554
Consolidation of multifamily ABS		$ (3,706,789)	$ 1,408,002
Deconsolidation, realized market valuation gain (loss)	$ (72,000)
Number of partnerships consolidated | partnership				2
VIE, ownership interest rate		80.00%
MSRs		$ 242,059			$ 151,582
Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Number of securitization entities to which asset transferred (in entities) | entity		51
Legacy Sequoia | Variable Interest Entity, Primary Beneficiary
Variable Interest Entity [Line Items]
Estimated fair value of investments		$ 5,000
Sequoia Choice | Variable Interest Entity, Primary Beneficiary
Variable Interest Entity [Line Items]
Estimated fair value of investments		211,000
Freddie Mac SLST | Variable Interest Entity, Primary Beneficiary
Variable Interest Entity [Line Items]
Estimated fair value of investments		418,000
ABS issued, net, Amortized cost		210,000
Freddie Mac K-Series | Variable Interest Entity, Primary Beneficiary
Variable Interest Entity [Line Items]
Estimated fair value of investments		27,000
CoreVest | Variable Interest Entity, Primary Beneficiary
Variable Interest Entity [Line Items]
Estimated fair value of investments		232,000
Servicing Investment | Variable Interest Entity, Primary Beneficiary
Variable Interest Entity [Line Items]
MSRs		$ 65,000