Borrowings - Note Payable (Details) - USD ($)			3 Months Ended
	Aug. 10, 2018	Feb. 28, 2018	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019	Nov. 01, 2018	Feb. 01, 2018
During the period:
Total interest expense			$ 15,349,000	$ 17,995,000
Carrying value:
Notes payable			1,294,514,000		$ 1,294,070,000
Unused amount			1,403,205,000		908,320,000
Note Payable
During the period:
Average balance			$ 1,300,000,000	$ 1,300,000,000
Weighted-average interest rate (as a percent)			4.43%	5.25%
Total interest expense			$ 14,846,000	$ 17,510,000
Maximum daily amount outstanding			1,300,000,000	1,300,000,000
Carrying value:
Unpaid principal balance			1,300,000,000		1,300,000,000
Unamortized debt issuance costs			(5,486,000)		(5,930,000)
Notes payable			$ 1,294,514,000		$ 1,294,070,000
Weighted-average interest rate (as a percent)			4.38%		4.46%
Amortization of Financing Costs			$ 445,000	$ 734,000
Note Payable | Revolving credit agreement
Short-term debt
Maximum loan amount						$ 150,000,000
Note Payable | LIBOR
Notes payable
Maximum loan amount	$ 650,000,000	$ 650,000,000
Description of variable rate	one-month LIBOR	one-month LIBOR
Interest rate spread	2.65%	2.85%
Note Payable | Credit Suisse AG
Short-term debt
Maximum loan amount							$ 400,000,000
Note Payable | Mortgage servicing rights
Carrying value:
Assets pledged to secure			2,117,619,000		$ 2,861,442,000
Note Payable | Servicing advances
Carrying value:
Assets pledged to secure			$ 182,531,000		$ 207,460,000