Supplementary Financial Information - Expenses as a Percent (unaudited) (Details) - USD ($) $ in Thousands		3 Months Ended								6 Months Ended		12 Months Ended
	Aug. 12, 2015	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Jun. 30, 2016	Dec. 31, 2016	Dec. 31, 2015
Total revenues (as a percent)		100.00%	100.00%	100.00%	100.00%	100.00%	100.00%	100.00%	100.00%
Expenses (as a percent)
Compensation and employee benefits (as a percent)		35.90%	43.30%	40.10%	42.10%	23.10%	38.40%	38.40%	38.30%
Transaction processing (as a percent)		20.50%	20.00%	18.30%	18.30%	9.00%	18.00%	17.90%	16.40%
Occupancy and equipment (as a percent)		12.00%	13.30%	11.70%	11.20%	6.40%	11.80%	10.30%	9.60%
Telecommunications and data processing services (as a percent)		11.10%	13.20%	12.30%	11.80%	5.80%	10.60%	9.30%	8.50%
Other general and administrative (as a percent)		17.40%	36.40%	21.60%	19.00%	8.90%	18.20%	30.20%	11.90%
Restructuring charges (as a percent)		4.40%		3.60%
Interest expense (as a percent)		0.50%	0.50%	0.50%	0.40%	0.20%	0.40%	0.30%	0.30%
Total expenses (as a percent)		101.80%	126.70%	108.10%	102.80%	53.40%	97.40%	106.40%	85.00%
Income (loss) before income tax expense		(1.80%)	(26.70%)	(8.10%)	(2.80%)	46.60%	2.60%	(6.40%)	15.00%
Income tax expense (as a percent)		(6.60%)	(3.70%)	(3.70%)	(0.90%)	10.00%	0.40%	0.90%	3.70%
Net income (loss) (as a percent)		4.80%	(23.00%)	(4.40%)	(1.90%)	36.60%	2.20%	(7.30%)	11.30%
Estimated Litigation Liability		$ 24,450								$ 24,450		$ 24,450
Legal and other settlement related costs													$ 4,900
Restructuring charge		5,265		$ 4,355								9,620
Decrease in income tax reserve due to resolution of uncertain tax positions		7,300								7,300		7,300
Aggregate amount of final settlement in connection with SEC's investigation							$ 20,300
Settlement agreement for termination of Former CEO
Expenses (as a percent)
Legal and other settlement related costs											$ 2,700	1,800
Litigation settlement amount												6,600
Net settlement charge											4,800	4,800
Insurance recovery		500		500						500	500	500
Proceeds from insurer to settle claims			$ 900								$ 900	900
Settlement Agreement With SEC Involving Prereleased ADRs
Expenses (as a percent)
Estimated Litigation Liability		24,500								24,500		$ 24,500
Legal and other settlement related costs		1,300	1,600							$ 2,900
Litigation settlement amount		2,400	$ 22,100
Settlement with S E C for Investigation into Proprietary Trading Pilot
Expenses (as a percent)
Legal and other settlement related costs							$ 2,600	$ 2,300
Litigation settlement amount	$ 20,300
Aggregate amount of final settlement in connection with SEC's investigation								$ 20,300
Employee separation and related costs
Expenses (as a percent)
Restructuring charge		$ 5,300
Reduction in trading and sales organizations and closing matched-book securities operations
Expenses (as a percent)
Restructuring charge				$ 4,300