Interest Income And Interest Expense (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Interest Income Expense Net [Abstract]
Bank loans, net	$ 68,389	$ 48,729	$ 132,024	$ 93,489
Investment securities	63,535	46,385	118,438	88,051
Margin balances	12,306	9,022	23,256	17,205
Financial instruments owned	4,983	4,346	9,912	8,582
Other	5,208	469	8,525	2,577
Total interest income	154,421	108,951	292,155	209,904
Senior notes	11,122	8,140	22,240	16,280
Bank deposits	15,343	1,887	23,473	3,654
Federal Home Loan Bank advances	3,095	2,797	6,347	4,516
Other	7,719	3,820	11,672	8,090
Total interest expense	$ 37,279	$ 16,644	$ 63,732	$ 32,540