Commitments and Contingencies - Legal Matters (Details) $ in Thousands					3 Months Ended		12 Months Ended
	Jan. 12, 2017 USD ($)	Aug. 12, 2016 item	Nov. 27, 2015 item	Aug. 12, 2015 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Dec. 31, 2017 item
Contingencies – Legal Matters
Accrued settlement costs					$ 24,450
Minimum
Contingencies – Legal Matters
Number of lawsuits proceedings may have a material impact on the results of operations for any particular period | item							1
Settlement Agreement With SEC Involving Prereleased ADRs
Contingencies – Legal Matters
Payment of settlement amount	$ 24,500
Disgorgement payment	15,100
Prejudgment interest	1,900
Civil penalty	$ 7,500
Accrued settlement costs					24,500
Litigation Settlement, Amount					$ (2,400)	$ (22,100)
Settlement with S E C for Investigation into Proprietary Trading Pilot
Contingencies – Legal Matters
Disgorgement payment				$ 2,100
Prejudgment interest				250
Civil penalty				18,000
Litigation Settlement, Amount				$ 20,300
Number of putative class action lawsuits | item		2
Case of derivative action captioned Watterson v. Gasser et al.
Contingencies – Legal Matters
Number of current or former officers and directors of the company got filed by the company | item			11