Revenues from contracts with customers (Disaggregation of Revenue) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 220,720		$ 439,436
Interest	13,056	$ 12,829	25,283	$ 23,394
Principal transactions, net	6,400	5,302	9,126	10,675
Other	2,380		3,241
Total other sources of revenue	21,836		37,650
Total revenue	242,556	215,884	477,086	429,145
Commissions from sales and trading
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	72,177		144,467
Mutual fund income
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	10,673		21,790
Advisory fees
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	77,270		154,818
Investment banking - capital markets
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	19,665		38,431
Investment banking - advisory
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	8,239		17,683
Bank deposit sweep income
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	28,853		54,150
Other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	3,843		8,097
Private Client
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	145,060		290,243
Interest	9,514		18,351
Principal transactions, net	212		132
Other	1,767		1,921
Total other sources of revenue	11,493		20,404
Total revenue	156,553	140,252	310,647	277,641
Private Client | Commissions from sales and trading
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	39,093		79,371
Private Client | Mutual fund income
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	10,441		21,268
Private Client | Advisory fees
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	59,774		119,910
Private Client | Investment banking - capital markets
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	3,469		7,839
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	28,853		54,150
Private Client | Other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	3,430		7,705
Asset Management
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	17,710		35,349
Interest	(4)		1
Principal transactions, net	0		0
Other	0		0
Total other sources of revenue	(4)		1
Total revenue	17,706	19,304	35,350	37,970
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	222		505
Asset Management | Advisory fees
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	17,485		34,838
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	3		6
Capital markets
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	57,922		113,787
Interest	3,263		6,468
Principal transactions, net	6,989		9,381
Other	32		99
Total other sources of revenue	10,284		15,948
Total revenue	68,206	$ 53,707	129,735	$ 109,610
Capital markets | Commissions from sales and trading
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	33,022		65,019
Capital markets | Mutual fund income
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	5		7
Capital markets | Advisory fees
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	3		54
Capital markets | Investment banking - capital markets
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	16,196		30,592
Capital markets | Investment banking - advisory
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	8,239		17,683
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	457		432
Corporate and Other [Member]
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	28		57
Interest	283		463
Principal transactions, net	(801)		(387)
Other	581		1,221
Total other sources of revenue	63		1,297
Total revenue	91		1,354
Corporate and Other [Member] | Commissions from sales and trading
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	62		77
Corporate and Other [Member] | Mutual fund income
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	5		10
Corporate and Other [Member] | Advisory fees
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	8		16
Corporate and Other [Member] | Investment banking - capital markets
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0		0
Corporate and Other [Member] | Investment banking - advisory
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0		0
Corporate and Other [Member] | Bank deposit sweep income
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0		0
Corporate and Other [Member] | Other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ (47)		$ (46)