Borrowings - Interest Expense on Broker-Dealer Facilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Line of Credit Facility [Line Items]
Interest expense	$ 121,859	$ 71,800	$ 64,107
Broker-dealer credit facilities
Line of Credit Facility [Line Items]
Interest expense	2,045	2,100	1,719
Uncommitted facility
Line of Credit Facility [Line Items]
Interest expense	1,591	1,794	1,667
Committed facility
Line of Credit Facility [Line Items]
Interest expense	$ 454	$ 306	$ 52