Borrowings - Mortgage Loan Participation and Sale Agreement (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
During the period:
Total interest expense	$ 999,000	$ 2,304,000	$ 3,870,000	$ 7,034,000
Carrying value:
Mortgage loan participation and sale agreement secured by mortgage loan participation certificates	$ 535,063,000		$ 535,063,000		$ 497,948,000
Weighted average interest rate (as a percent)	1.85%		1.85%		3.29%
Fair value of loans pledged to secure	$ 558,023,000		$ 558,023,000		$ 523,349,000
Mortgage Loan Participation and Sale Agreement member
During the period:
Average balance	$ 235,713,000	$ 258,169,000	$ 227,460,000	$ 249,023,000
Weighted-average interest rate (as a percent)	1.40%	3.36%	1.98%	3.55%
Total interest expense	$ 999,000	$ 2,304,000	$ 3,870,000	$ 7,034,000
Carrying value:
Unpaid principal balance of mortgage loan participation and sale agreement secured by mortgage loan participation certificates	535,378,000		535,378,000		497,948,000
Unamortized debt issuance costs	(315,000)		(315,000)
Mortgage loan participation and sale agreement secured by mortgage loan participation certificates	$ 535,063,000		$ 535,063,000		$ 497,948,000
Weighted average interest rate (as a percent)	1.40%		1.40%		3.05%
Fair value of loans pledged to secure	$ 558,023,000		$ 558,023,000		$ 523,349,000
Amortization of debt issuance costs	172,000	135,000	490,000	405,000
Maximum | Mortgage Loan Participation and Sale Agreement member
Carrying value:
Mortgage loan participation and sale agreement secured by mortgage loan participation certificates	$ 538,074,000	$ 524,095,000	$ 540,977,000	$ 548,038,000