Contingencies - Legal Matters (Details) - USD ($) $ in Thousands		3 Months Ended	6 Months Ended
	Aug. 12, 2015	Jun. 30, 2018	Jun. 30, 2018
Contingencies – Legal Matters
Litigation settlement amount			$ 18,000
Accrued costs for potential class action lawsuit settlement		$ 18,000	18,000
Expense incurred as potential settlement with SEC		12,000	12,000
Legal expenses		200	200
Settlement with SEC regard to the operational features of U.S. POSIT
Contingencies – Legal Matters
Expense incurred as potential settlement with SEC		12,000	$ 12,000
Legal expenses		$ 200
Settlement with S E C for Investigation into Proprietary Trading Pilot
Contingencies – Legal Matters
Payment of settlement amount	$ 20,300
Civil penalty	18,000
Disgorgement payment	2,100
Prejudgment interest	$ 250