Revenues from contracts with customers (Disaggregation of Revenue) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 257,461	$ 214,528	$ 730,008	$ 664,473
Interest	(7,540)	(12,344)	(24,650)	(38,621)
Principal transactions, net	7,703	7,606	18,899	22,089
Other	3,555	315	2,202	12,315
Total other sources of revenue	18,798	20,265	45,751	73,025
Total revenue	(276,259)	(234,793)	(775,759)	(737,498)
Commissions from sales and trading
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	84,134	68,572	270,687	209,133
Mutual Fund Income [Member]
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	8,107	10,055	26,439	29,799
Advisory fees
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	88,595	80,887	250,740	235,241
Investment banking - capital markets
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	35,539	11,330	90,373	41,633
Investment banking - advisory
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	30,706	10,468	47,786	40,214
Bank deposit sweep income
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	4,619	28,894	30,567	94,692
Other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	5,761	4,322	13,416	13,761
Private Client
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	131,089	143,301	403,295	436,279
Interest	(5,939)	(8,652)	(18,753)	(27,699)
Principal transactions, net	1,223	70	1,164	2,292
Other	2,846	31	1,128	11,239
Total other sources of revenue	10,008	8,753	21,045	41,230
Total revenue	(141,097)	(152,054)	(424,340)	(477,509)
Private Client | Commissions from sales and trading
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	40,742	35,994	129,652	110,496
Private Client | Mutual Fund Income [Member]
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	8,097	10,050	26,414	29,788
Private Client | Advisory fees
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	67,949	62,510	193,297	181,634
Private Client | Investment banking - capital markets
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	3,962	2,557	11,382	9,568
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	4,619	28,894	30,567	94,692
Private Client | Other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	5,720	3,296	11,983	10,101
Asset Management
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	20,634	18,368	57,414	53,569
Interest	(5)	0	0	0
Principal transactions, net	0	0	0	0
Other	3	0	9	7
Total other sources of revenue	(2)	0	9	7
Total revenue	(20,632)	(18,368)	(57,423)	(53,576)
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	0	3	(5)
Asset Management | Advisory fees
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	20,634	18,366	57,411	53,572
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	2	0	2
Capital markets
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	105,706	52,261	269,117	172,201
Interest	(1,539)	(3,272)	(5,390)	(9,566)
Principal transactions, net	6,357	8,515	19,853	24,984
Other	687	20	741	97
Total other sources of revenue	8,583	11,807	25,984	34,647
Total revenue	(114,289)	(64,068)	(295,101)	(206,848)
Capital markets | Commissions from sales and trading
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	43,389	32,569	141,013	98,621
Capital markets | Mutual Fund Income [Member]
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	2	1	7	3
Capital markets | Advisory fees
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	2	2	9
Capital markets | Investment banking - capital markets
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	31,577	8,773	78,991	32,065
Capital markets | Investment banking - advisory
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	30,706	10,468	47,786	40,214
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	32	448	1,318	1,289
Corporate/Other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	32	598	182	2,424
Interest	(67)	(420)	(507)	(1,356)
Principal transactions, net	123	(979)	(2,118)	(5,187)
Other	19	264	324	972
Total other sources of revenue	209	(295)	(1,287)	(2,859)
Total revenue	(241)	(303)	(1,105)	435
Corporate/Other | Commissions from sales and trading
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	3	9	22	16
Corporate/Other | Mutual Fund Income [Member]
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	8	4	15	13
Corporate/Other | Advisory fees
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	12	9	30	26
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	$ 9	$ 576	$ 115	$ 2,369