Revenues from contracts with customers (Disaggregation of Revenue) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 237,839	$ 218,920
Interest	10,890	12,727
Principal transactions, net	(868)	11,438
Other	(13,091)	8,685
Total other sources of revenue	(3,069)	32,850
Total revenue	234,770	251,770
Commissions from sales and trading
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	93,412	69,795
Mutual fund income
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	9,837	9,614
Advisory fees
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	86,164	73,647
Investment banking - capital markets
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	15,892	11,342
Investment banking - advisory
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	9,836	16,701
Bank deposit sweep income
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	18,826	33,968
Other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	3,872	3,853
Private Client
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	149,722	144,130
Interest	7,680	9,408
Principal transactions, net	(2,715)	1,684
Other	(13,269)	8,305
Total other sources of revenue	(8,304)	19,397
Total revenue	141,418	163,527
Private Client | Commissions from sales and trading
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	47,105	37,477
Private Client | Mutual fund income
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	9,827	9,614
Private Client | Advisory fees
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	66,883	57,044
Private Client | Investment banking - capital markets
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	3,950	2,749
Private Client | Investment banking - advisory
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Private Client | Bank deposit sweep income
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	18,826	33,968
Private Client | Other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	3,131	3,278
Asset Management
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	19,273	16,583
Interest	0	0
Principal transactions, net	0	0
Other	3	3
Total other sources of revenue	3	3
Total revenue	19,276	16,586
Asset Management | Commissions from sales and trading
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Asset Management | Mutual fund income
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	3	(6)
Asset Management | Advisory fees
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	19,270	16,589
Asset Management | Investment banking - capital markets
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Asset Management | Investment banking - advisory
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Asset Management | Bank deposit sweep income
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Asset Management | Other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Capital markets
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	68,710	57,935
Interest	2,824	2,835
Principal transactions, net	3,984	10,157
Other	24	34
Total other sources of revenue	6,832	13,026
Total revenue	75,542	70,961
Capital markets | Commissions from sales and trading
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	46,287	32,316
Capital markets | Mutual fund income
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	3	1
Capital markets | Advisory fees
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	2	5
Capital markets | Investment banking - capital markets
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	11,942	8,593
Capital markets | Investment banking - advisory
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	9,836	16,701
Capital markets | Bank deposit sweep income
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Capital markets | Other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	640	319
Corporate/Other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	134	272
Interest	386	484
Principal transactions, net	(2,137)	(403)
Other	151	343
Total other sources of revenue	(1,600)	424
Total revenue	(1,466)	696
Corporate/Other | Commissions from sales and trading
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	20	2
Corporate/Other | Mutual fund income
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	4	5
Corporate/Other | Advisory fees
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	9	9
Corporate/Other | Investment banking - capital markets
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Corporate/Other | Investment banking - advisory
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Corporate/Other | Bank deposit sweep income
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Corporate/Other | Other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 101	$ 256