Supplementary Financial Information - Expenses as a Percent (unaudited) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Aug. 31, 2015
Supplementary Financial Information (unaudited)
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)	23.10%	38.40%	38.40%	38.30%	39.90%	38.90%	38.10%	37.20%
Transaction processing (as a percent)	9.00%	18.00%	17.90%	16.40%	16.30%	16.00%	14.50%	14.90%
Occupancy and equipment (as a percent)	6.40%	11.80%	10.30%	9.60%	9.90%	11.10%	10.80%	11.00%
Telecommunications and data processing services (as a percent)	5.80%	10.60%	9.30%	8.50%	8.70%	9.60%	9.10%	9.20%
Other general and administrative (as a percent)	8.90%	18.20%	30.20%	11.90%	12.90%	15.00%	15.00%	13.90%
Interest expense (as a percent)	0.20%	0.40%	0.30%	0.30%	0.40%	0.40%	0.40%	0.40%
Total expenses (as a percent)	53.40%	97.40%	106.40%	85.00%	88.10%	91.00%	87.90%	86.60%
Income (loss) before income tax expense	46.60%	2.60%	(6.40%)	15.00%	11.90%	9.00%	12.10%	13.40%
Income tax expense (as a percent)	10.00%	0.40%	0.90%	3.70%	3.20%	0.50%	2.70%	3.50%
Net income (loss) (as a percent)	36.60%	2.20%	(7.30%)	11.30%	8.70%	8.50%	9.40%	9.90%
Aggregate amount of final settlement in connection with SEC's investigation			$ 20.3							$ 20.3
Legal and other settlement related costs			$ 2.3						$ 4.9