Interest Income And Interest Expense (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Interest Income Expense Net [Abstract]
Bank loans, net									$ 300,541	$ 206,084	$ 127,923
Investment securities									252,200	187,731	107,454
Margin balances									49,515	37,218	32,147
Financial instruments owned									21,407	17,563	18,965
Other									22,786	5,785	7,843
Total interest income									646,449	454,381	294,332
Bank deposits									82,256	12,661	7,331
Senior notes									44,610	35,338	36,217
Federal Home Loan Bank advances									15,173	8,305	6,777
Other									28,037	13,726	16,549
Total interest expense	$ 57,876	$ 48,468	$ 37,279	$ 26,453	$ 19,865	$ 17,625	$ 16,644	$ 15,896	$ 170,076	$ 70,030	$ 66,874