Borrowings - Broker-Dealer Credit Facilities Carrying Values, Net (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Line of Credit Facility [Line Items]
Borrowing Outstanding	$ 1,957,225
Outstanding Borrowings, net	$ 1,917,866	$ 907,037
Uncommitted facility
Line of Credit Facility [Line Items]
Interest Rate	2.55%	3.40%
Financing Available	$ 200,000	$ 200,000
Borrowing Outstanding	30,000	10,000
Deferred Debt Issuance Cost	(2,100)	(832)
Outstanding Borrowings, net	$ 27,900	$ 9,168
Committed facility
Line of Credit Facility [Line Items]
Interest Rate	3.01%	3.75%
Financing Available	$ 600,000	$ 500,000
Borrowing Outstanding	0	7,000
Deferred Debt Issuance Cost	0	(1,040)
Outstanding Borrowings, net	0	5,960
Broker-dealer credit facilities
Line of Credit Facility [Line Items]
Financing Available	800,000	700,000
Borrowing Outstanding	30,000	17,000
Deferred Debt Issuance Cost	(2,100)	(1,872)
Outstanding Borrowings, net	$ 27,900	$ 15,128