Interest Income And Interest Expense (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Interest Income Expense Net [Abstract]
Bank loans, net	$ 78,766	$ 54,006	$ 210,790	$ 147,495
Investment securities	66,825	48,072	185,263	136,123
Margin balances	13,066	9,919	36,322	27,124
Financial instruments owned	5,733	4,371	15,645	12,953
Other	5,370	1,494	13,895	4,071
Total interest income	169,760	117,862	461,915	327,766
Bank deposits	25,856	4,770	49,329	8,424
Senior notes	11,248	8,655	33,488	24,935
Federal Home Loan Bank advances	4,232	1,255	10,579	5,771
Other	7,132	2,945	18,804	11,035
Total interest expense	$ 48,468	$ 17,625	$ 112,200	$ 50,165