Borrowings - Note Payable (Details) - USD ($)			3 Months Ended		9 Months Ended
	Aug. 10, 2018	Feb. 28, 2018	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Nov. 01, 2018	Feb. 01, 2018
During the period:
Total interest expense			$ 17,525,000	$ 21,369,000	$ 53,559,000	$ 55,939,000
Carrying value:
Notes payable			1,293,625,000		1,293,625,000		$ 1,292,291,000
Notes payable
Repayments of notes payable						900,000,000
Note Payable
During the period:
Average balance			$ 1,300,000,000	$ 1,234,783,000	$ 1,300,000,000	$ 1,125,458,000
Weighted-average interest rate (as a percent)			5.11%	5.07%	5.21%	5.29%
Total interest expense			$ 17,525,000	$ 21,369,000	$ 53,559,000	$ 55,939,000
Maximum daily amount outstanding			1,300,000,000	1,300,000,000	1,300,000,000	1,300,000,000
Carrying value:
Unpaid principal balance			1,300,000,000		1,300,000,000		1,300,000,000
Unamortized issuance costs			(6,375,000)		(6,375,000)		(7,709,000)
Notes payable			$ 1,293,625,000		$ 1,293,625,000		$ 1,292,291,000
Weighted-average interest rate (as a percent)			4.90%		4.90%		5.07%
Unused amount			$ 150,000,000		$ 150,000,000		$ 150,000,000
Amortization of Financing Costs			900,000	2,000,000	2,700,000	2,800,000
Notes payable
Debt issuance costs				3,400,000		8,000,000
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			2,489,481,000		2,489,481,000		2,807,333,000
Note Payable | Cash.
Carrying value:
Assets pledged to secure			101,773,000		101,773,000		108,174,000
Note Payable | Servicing advances
Carrying value:
Assets pledged to secure			181,747,000		181,747,000		162,895,000
Mortgage Loan Participation and Sale Agreement member
During the period:
Average balance			258,169,000	289,008,000	249,023,000	250,599,000
Carrying value:
Unamortized issuance costs							$ (215,000)
Amortization of Financing Costs			$ 135,000	$ 92,000	$ 405,000	$ 475,000