Regulatory Requirements Regulatory (Tables)

12 Months Ended

Dec. 31, 2017

Regulatory [Abstract]

Computation of Net Capital under Securities and Exchange Commission Regulation [Table Text Block]

Subsidiary	Net Capital		Minimum Net Capital Requirement		Excess Net Capital
	(dollars in millions)
Cowen and Company	$	40.2	$	1.0	$	39.2
Cowen Execution	$	111.4	$	2.7	$	108.7
ATM Execution	$	1.0	$	0.3	$	0.7
Cowen Prime	$	12.3	$	0.3	$	12.0
Westminster	$	7.6	$	0.3	$	7.3
Ramius UK	$	0.2	$	0.1	$	0.1
Cowen International Ltd	$	11.5	$	7.1	$	4.4
Cowen Execution Ltd	$	3.8	$	2.6	$	1.2