Borrowings - Note Payable (Details) - USD ($)					3 Months Ended		9 Months Ended
	Aug. 10, 2018	Feb. 28, 2018	Aug. 10, 2017	Feb. 16, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
During the period:
Total interest expense					$ 21,369,000	$ 11,747,000	$ 55,939,000	$ 24,746,000
Carrying value:
Notes payable					1,291,847,000		1,291,847,000		$ 891,505,000
Notes payable
Repayments of notes payable							900,000,000	186,935,000
Debt issuance costs							3,400,000
Note Payable
During the period:
Average balance					$ 1,234,783,000	$ 689,417,000	$ 1,125,458,000	$ 483,370,000
Weighted-average interest rate (as a percent)					5.07%	5.85%	5.29%	5.87%
Total interest expense					$ 21,369,000	$ 11,747,000	$ 55,939,000	$ 24,746,000
Maximum daily amount outstanding					1,300,000,000	890,879,000	1,300,000,000	891,011,000
Carrying value:
Unpaid principal balance					1,300,000,000		1,300,000,000		900,006,000
Unamortized issuance costs					(8,153,000)		(8,153,000)		(8,501,000)
Notes payable					$ 1,291,847,000		$ 1,291,847,000		$ 891,505,000
Weighted-average interest rate (as a percent)					4.82%		4.82%		5.66%
Unused amount					$ 150,000,000		$ 150,000,000		$ 280,000,000
Amortization of Financing Costs					5,200,000	$ 1,200,000	10,300,000	$ 3,200,000
Non-utilization fees					179,000		562,000
Notes payable
Repayments of notes payable	$ 500,000,000	$ 400,000,000
Debt issuance costs					4,600,000		4,600,000
Note Payable | Revolving credit agreement
Short-term Debt [Line Items]
Maximum loan amount					150,000,000		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 | Mortgage servicing rights
Carrying value:
Assets pledged to secure					2,539,575,000		2,539,575,000		2,098,067,000
Note Payable | Cash.
Carrying value:
Assets pledged to secure					81,640,000		81,640,000		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					$ 102,222,000		$ 102,222,000		$ 114,643,000