Borrowings - Note Payable (Details) - USD ($)					12 Months Ended
	Aug. 10, 2018	Feb. 28, 2018	Aug. 10, 2017	Feb. 16, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Feb. 01, 2018
During the period:
Total interest expense					$ 73,610,000	$ 39,369,000	$ 8,688,000
Carrying value:
Notes payable					1,292,291,000	891,505,000
Notes payable
Repayments of notes payable					900,000,000	186,935,000	33,661,000
Note Payable
During the period:
Average balance					$ 1,169,452,000	$ 586,135,000	$ 108,475,000
Weighted-average interest rate (as a percent)					5.29%	5.86%	5.13%
Total interest expense					$ 73,610,000	$ 39,369,000	$ 8,688,000
Maximum daily amount outstanding					1,300,000,000	900,006,000	153,849,000
Carrying value:
Unpaid principal balance					1,300,000,000	900,006,000
Unamortized issuance costs					(7,709,000)	(8,501,000)
Notes payable					$ 1,292,291,000	$ 891,505,000
Weighted-average interest rate (as a percent)					5.07%	5.66%
Unused amount					$ 150,000,000	$ 280,000,000
Amortization of Financing Costs					11,700,000	4,500,000	$ 3,000,000
Notes payable
Repayments of notes payable	$ 500,000,000	$ 400,000,000
Debt issuance costs					4,600,000
Note Payable | Revolving credit agreement
Short-term Debt [Line Items]
Maximum loan amount					150,000,000
Note Payable | LIBOR
Notes payable
Maximum loan amount	$ 650,000,000	$ 650,000,000	$ 500,000,000	$ 400,000,000
Description of variable rate	one-month LIBOR	one-month LIBOR	one-month LIBOR	one-month LIBOR
Interest rate spread	2.65%	2.85%	4.00%	4.75%
Note Payable | Credit Suisse AG
Short-term Debt [Line Items]
Maximum loan amount								$ 400,000,000
Carrying value:
Notes payable					0
Note Payable | Mortgage servicing rights
Carrying value:
Assets pledged to secure					2,807,333,000	2,098,067,000
Note Payable | Cash.
Carrying value:
Assets pledged to secure					108,174,000	20,765,000
Note Payable | Carried interest
Carrying value:
Assets pledged to secure						8,552,000
Note Payable | Servicing advances
Carrying value:
Assets pledged to secure					162,895,000	$ 114,643,000
Loan and Security Agreement
Notes payable
Debt issuance costs					$ 3,400,000