Commitments, Guarantees, And Contingencies (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Loss Contingencies [Line Items]
Customer-owned securities pledged	$ 278.7	$ 39.2
Minimum margin deposit requirements	27.4
Outstanding committed capital to certain entities	23.8
Rent expense	93.6	$ 78.6	$ 74.2
Business Development Corporations [Member]
Loss Contingencies [Line Items]
Outstanding committed capital to certain entities	12.5
Options Clearing Corporation [Member]
Loss Contingencies [Line Items]
Customer-owned securities pledged	$ 82.2