Borrowings - Broker-Dealer Credit Facilities Carrying Values, Net (Details) - USD ($)	Dec. 31, 2018	Dec. 31, 2017	Nov. 03, 2017	Nov. 02, 2017
Line of Credit Facility [Line Items]
Borrowing Outstanding	$ 931,908,000
Outstanding Borrowings, net	$ 907,037,000	$ 1,388,548,000
Uncommitted facility
Line of Credit Facility [Line Items]
Interest Rate	3.40%	2.42%
Financing Available	$ 200,000,000	$ 150,000,000
Borrowing Outstanding	10,000,000	25,000,000
Deferred Debt Issuance Cost	(832,000)	0
Outstanding Borrowings, net	$ 9,168,000	$ 25,000,000
Revolving credit facility
Line of Credit Facility [Line Items]
Interest Rate	3.75%	2.81%
Financing Available	$ 500,000,000	$ 500,000,000
Borrowing Outstanding	7,000,000	7,000,000
Deferred Debt Issuance Cost	(1,040,000)	(4,117,000)
Outstanding Borrowings, net	5,960,000	2,883,000
Broker-dealer credit facilities
Line of Credit Facility [Line Items]
Financing Available	700,000,000	650,000,000	$ 500,000,000	$ 25,000,000
Borrowing Outstanding	17,000,000	32,000,000
Deferred Debt Issuance Cost	(1,872,000)	(4,117,000)
Outstanding Borrowings, net	$ 15,128,000	$ 27,883,000