Commitments and Contingencies - Additional Information (Details) $ in Thousands			3 Months Ended			9 Months Ended		12 Months Ended		48 Months Ended
	Sep. 30, 2019 USD ($) lease	Jul. 15, 2010 plaintiff	Sep. 30, 2019 USD ($) lease	Dec. 31, 2018 USD ($) partnership	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) loan lease certificate repurchase_request	Sep. 30, 2018 USD ($)	Dec. 31, 2007 certificate	Dec. 31, 2004 certificate	Dec. 31, 2007 certificate	Mar. 01, 2019 USD ($)	Jan. 01, 2019 USD ($) lease	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Loss Contingencies [Line Items]
Lessee, number of leases | lease	5		5			5
Present value of remaining lease payments	$ 15,490		$ 15,490			$ 15,490						$ 15,000
Operating lease expense						$ 2,000	$ 2,000
Number of noncancelable leases | lease												4
Number of leases qualified as short-term lease | lease						1
Lease liability	12,570		12,570	$ 0		$ 12,570
Right-of-use asset	$ 11,076		$ 11,076	$ 0		$ 11,076
Number of leases without implicit interest rate | lease						4
Weighted average remaining lease term	8 years		8 years			8 years
Discount rate	5.30%		5.30%			5.30%
Number of partnerships, committed to fund | partnership				2
Commitment to fund partnership	$ 71,000
Contingent consideration	25,167		$ 25,167	$ 0		$ 25,167
Other income related to risk sharing agreement			905		$ 907	2,351	2,706
Guarantee obligations	15,016		15,016	16,711		15,016
Guarantee obligations, credit reserve	5,000		5,000			5,000
Special Purpose Entities (SPEs) assets	48,000		48,000	47,000		48,000
Special Purpose Entities (SPEs) liabilities	15,000		15,000	17,000		15,000
Residential repurchase reserve	3,947		3,947	4,189		$ 3,947
Number of residential repurchase requests (in repurchase requests) | repurchase_request						10
Number of loans repurchased | loan						0
Residential repurchase provisions recorded						$ (200)	(200)
Aggregate amount of loss contingency reserves	2,000		2,000	2,000		$ 2,000
Morgan Stanley And Company
Loss Contingencies [Line Items]
Number of mortgage pass-through certificates issued (in certificates) | certificate						28
Sequoia Residential Funding
Loss Contingencies [Line Items]
Number of mortgage pass-through certificates issued (in certificates) | certificate								2	2	4
Number of certificated withdrawn | certificate									1	8
Schwab
Loss Contingencies [Line Items]
Number of other named defendants along with SRF (in plaintiffs) | plaintiff		26
Residential Loans
Loss Contingencies [Line Items]
Loan principal	904,000		904,000	1,030,000		$ 904,000
Loans held-for-investment, in foreclosure	2,000		2,000			2,000
Financing Receivables, Equal to Greater than 90 Days Past Due | Residential Loans
Loss Contingencies [Line Items]
Loan principal	700		700	$ 700		700
Other income
Loss Contingencies [Line Items]
Other income related to risk sharing agreement			1,000		1,000	2,000	3,000
Mortgage banking and investment activities
Loss Contingencies [Line Items]
Market valuation losses related to these investments			(100)		$ (100)	(200)	$ (500)
Guarantee Obligations
Loss Contingencies [Line Items]
Original unpaid balance of loans subject to risk sharing agreements													$ 3,190,000	$ 3,190,000
Potential future payments on loans	44,000		44,000			44,000
Loan principal	$ 1,660,000		$ 1,660,000			$ 1,660,000
Weighted average original Fair Isaac Corporation (FICO) score	759		759			759
Weighted average original loan-to-value (LTV)	76.00%		76.00%			76.00%
Guarantee Obligations | Financing Receivables, Equal to Greater than 90 Days Past Due
Loss Contingencies [Line Items]
Balance of loans 90 days or more delinquent	$ 7,000		$ 7,000			$ 7,000
Residential | Sequoia | FHLB Seattle
Loss Contingencies [Line Items]
Principal value	133,000		133,000			133,000
Debt instrument principal payment amount						128,000
Debt instrument interest payment amount						12,000
Residential | Sequoia | Schwab
Loss Contingencies [Line Items]
Principal value	15,000		15,000			15,000
Principal balance of securities	14,000		14,000			14,000
Debt instrument interest amount	1,000		1,000			1,000
Financial Guarantee
Loss Contingencies [Line Items]
Commitment to fund partnership	49,000
Guarantor obligations, current carrying value	100		100			100
Guarantor obligations, maximum exposure, undiscounted	135,000		135,000			135,000
ASU 2016-02
Loss Contingencies [Line Items]
Lease liability	13,000		13,000			13,000
Right-of-use asset	11,000		11,000			11,000
Commitment To Fund Residential Bridge Loan
Loss Contingencies [Line Items]
Commitments to fund temporary advances	67,000		67,000			67,000
Commitment To Fund Temporary Advances On Residential Bridge Loans
Loss Contingencies [Line Items]
Commitments to fund temporary advances	65,000		65,000			65,000
5 Arches, LLC
Loss Contingencies [Line Items]
Contingent consideration maximum amount											$ 27,000
Contingent consideration	25,000		25,000			25,000					$ 24,621
Contingent consideration expense			200			500
Home appreciation options
Loss Contingencies [Line Items]
Payments to acquire investments	11,000
Home appreciation options | Commitment To Fund Investment
Loss Contingencies [Line Items]
Commitments to fund temporary advances	$ 39,000		$ 39,000			$ 39,000