Principles of Consolidation - Additional Information (Details) $ in Thousands	3 Months Ended		12 Months Ended	93 Months Ended
	Sep. 30, 2019 USD ($)	Dec. 31, 2018 USD ($) partnership	Dec. 31, 2018 USD ($) partnership	Sep. 30, 2019 USD ($) entity
Variable Interest Entity [Line Items]
Number of partnerships consolidated | partnership		2	2
MSRs	$ 204,946	$ 286,778	$ 286,778	$ 204,946
Variable Interest Entity, Primary Beneficiary
Variable Interest Entity [Line Items]
VIE, ownership interest rate		80.00%
Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Number of securitization entities to which asset transferred (in entities) | entity				46
Gain (loss) on securitization of financial assets	4,000
Loan acquisition, aggregate amount acquired	39,000
Legacy Sequoia | Variable Interest Entity, Primary Beneficiary
Variable Interest Entity [Line Items]
Estimated fair value of investments	10,000			$ 10,000
Sequoia Choice | Variable Interest Entity, Primary Beneficiary
Variable Interest Entity [Line Items]
Estimated fair value of investments	259,000			259,000
Freddie Mac SLST | Variable Interest Entity, Primary Beneficiary
Variable Interest Entity [Line Items]
Estimated fair value of investments	456,000			456,000
Freddie Mac K-Series | Variable Interest Entity, Primary Beneficiary
Variable Interest Entity [Line Items]
Estimated fair value of investments	215,000			215,000
Servicing Investment | Variable Interest Entity, Primary Beneficiary
Variable Interest Entity [Line Items]
MSRs	$ 75,000			$ 75,000