Interest Income and Interest Expense (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Interest Income Expense Net [Abstract]
Bank loans, net	$ 95,436	$ 78,766	$ 284,541	$ 210,790
Investment securities	54,945	66,825	182,127	185,263
Margin balances	13,334	13,066	40,437	36,322
Financial instruments owned	5,668	5,733	18,395	15,645
Other	9,401	5,370	32,295	13,895
Total interest income	178,784	169,760	557,795	461,915
Bank deposits	24,815	25,856	82,228	49,329
Senior notes	11,121	11,248	33,365	33,488
Federal Home Loan Bank advances	1,146	4,232	5,735	10,579
Other	7,062	7,132	25,155	18,804
Total interest expense	$ 44,144	$ 48,468	$ 146,483	$ 112,200