Revenues from contracts with customers (Disaggregation of Revenue) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 234,709	$ 231,024	$ 472,548	$ 449,945
Interest	6,220	13,550	17,110	26,277
Principal transactions, net	12,064	3,045	11,196	14,483
Other	11,737	3,316	(1,354)	12,000
Total other sources of revenue	30,021	19,911	26,952	52,760
Total revenue	264,730	250,935	499,500	502,705
Commissions from sales and trading
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	93,142	70,766	186,554	140,561
Mutual Fund Income [Member]
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	8,495	10,130	18,332	19,744
Advisory fees
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	75,982	80,707	162,146	154,354
Investment banking - capital markets
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	38,942	18,961	54,834	30,303
Investment banking - advisory
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	7,244	13,045	17,080	29,746
Bank deposit sweep income
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	7,122	31,830	25,948	65,798
Other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	3,782	5,585	7,654	9,439
Private Client
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	122,484	148,848	272,206	292,978
Interest	5,134	9,639	12,814	19,047
Principal transactions, net	2,656	538	(59)	2,222
Other	11,551	2,903	(1,718)	11,208
Total other sources of revenue	19,341	13,080	11,037	32,477
Total revenue	141,825	161,928	283,243	325,455
Private Client | Commissions from sales and trading
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	41,805	37,026	88,910	74,502
Private Client | Mutual Fund Income [Member]
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	8,490	10,124	18,317	19,738
Private Client | Advisory fees
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	58,465	62,080	125,348	119,124
Private Client | Investment banking - capital markets
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	3,470	4,262	7,420	7,011
Private Client | Investment banking - advisory
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Private Client | Bank deposit sweep income
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	7,122	31,830	25,948	65,798
Private Client | Other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	3,132	3,526	6,263	6,805
Asset Management
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	17,507	18,618	36,780	35,201
Interest	5	0	5	0
Principal transactions, net	0	0	0	0
Other	3	4	6	7
Total other sources of revenue	8	4	11	7
Total revenue	17,515	18,622	36,791	35,208
Asset Management | Commissions from sales and trading
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Asset Management | Mutual Fund Income [Member]
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	1	3	(5)
Asset Management | Advisory fees
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	17,507	18,617	36,777	35,206
Asset Management | Investment banking - capital markets
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Asset Management | Investment banking - advisory
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Asset Management | Bank deposit sweep income
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Asset Management | Other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Capital markets
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	94,701	62,005	163,411	119,940
Interest	1,027	3,459	3,851	6,294
Principal transactions, net	9,512	6,312	13,496	16,469
Other	30	43	54	77
Total other sources of revenue	10,569	9,814	17,401	22,840
Total revenue	105,270	71,819	180,812	142,780
Capital markets | Commissions from sales and trading
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	51,337	33,736	97,624	66,052
Capital markets | Mutual Fund Income [Member]
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	2	1	5	2
Capital markets | Advisory fees
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	2	2	7
Capital markets | Investment banking - capital markets
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	35,472	14,699	47,414	23,292
Capital markets | Investment banking - advisory
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	7,244	13,045	17,080	29,746
Capital markets | Bank deposit sweep income
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Capital markets | Other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	646	522	1,286	841
Corporate/Other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	17	1,553	151	1,826
Interest	54	452	440	936
Principal transactions, net	(104)	(3,805)	(2,241)	(4,208)
Other	153	366	304	708
Total other sources of revenue	103	(2,987)	(1,497)	(2,564)
Total revenue	120	(1,434)	(1,346)	(738)
Corporate/Other | Commissions from sales and trading
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	4	20	7
Corporate/Other | Mutual Fund Income [Member]
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	3	4	7	9
Corporate/Other | Advisory fees
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	10	8	19	17
Corporate/Other | Investment banking - capital markets
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Corporate/Other | Investment banking - advisory
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Corporate/Other | Bank deposit sweep income
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Corporate/Other | Other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 4	$ 1,537	$ 105	$ 1,793