Business Changes and Developments - Intangible Assets Associated with Acquisitions (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 51,595	$ 54,060
Accumulated Amortization	32,018	24,690
Investment Banking [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	47,320	49,631
Accumulated Amortization	29,762	22,757
Investment Management [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	4,275	4,429
Accumulated Amortization	2,256	1,933
Client Related [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	45,830	48,141
Accumulated Amortization	29,332	22,696
Client Related [Member] | Investment Banking [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	42,000	44,311
Accumulated Amortization	27,355	21,110
Client Related [Member] | Investment Management [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	3,830	3,830
Accumulated Amortization	1,977	1,586
Non Compete Non Solicit Agreements [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount		154
Accumulated Amortization		124
Non Compete Non Solicit Agreements [Member] | Investment Banking [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount		0
Accumulated Amortization		0
Non Compete Non Solicit Agreements [Member] | Investment Management [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount		154
Accumulated Amortization		124
Other [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	5,765	5,765
Accumulated Amortization	2,686	1,870
Other [Member] | Investment Banking [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	5,320	5,320
Accumulated Amortization	2,407	1,647
Other [Member] | Investment Management [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	445	445
Accumulated Amortization	$ 279	$ 223