Segment and Geographic Information (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2012
Segment Reporting Information [Line Items]
Revenues	$ 10,655.7	$ 7,005.1	$ 8,452.9	$ 7,908.7	$ 8,927.5	$ 9,659.1	$ 11,769.3	$ 12,137.5	$ 34,022.4	$ 42,493.4	$ 66,693.4
Operating revenues	130.6	118.2	129.2	112.9	111.9	123.0	114.9	118.4	490.9	468.2	448.1
Revenues, Net of Interest Expense	86.7	76.0	85.9	73.4	70.0	81.3	75.1	83.3	322.0	309.7	306.2
Net Segment Contribution									249.2	228.7	226.5
Segment Income									128.8	113.0	113.2
Costs not allocated to operating segments									102.8	91.8	90.7
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest	7.8	4.0	10.3	3.9	(1.6)	7.1	1.7	14.0	26.0	21.2	22.5
Assets	3,039.7				2,848.0				3,039.7	2,848.0	2,953.0
Commodity and Risk Management [Member]
Segment Reporting Information [Line Items]
Revenues									224.0	202.0	230.7
Operating revenues									224.0	202.0	230.7
Revenues, Net of Interest Expense									180.5	163.0	194.7
Net Segment Contribution									132.6	119.7	138.9
Segment Income									67.3	57.1	75.2
Assets	1,400.9				1,005.1				1,400.9	1,005.1	1,120.4
Foreign Exchange Segment [Member]
Segment Reporting Information [Line Items]
Revenues									55.4	40.9	39.0
Operating revenues									55.4	40.9	39.0
Revenues, Net of Interest Expense									48.2	36.7	35.1
Net Segment Contribution									37.6	28.0	27.3
Segment Income									28.3	20.5	21.6
Assets	51.9				57.2				51.9	57.2	53.9
Securities Segment [Member]
Segment Reporting Information [Line Items]
Revenues									80.3	70.0	47.6
Operating revenues									80.3	70.0	47.6
Revenues, Net of Interest Expense									54.6	48.0	34.5
Net Segment Contribution									40.9	37.6	25.8
Segment Income									21.0	19.5	7.7
Assets	235.5				204.2				235.5	204.2	92.2
Clearing and Execution Services Segment [Member]
Segment Reporting Information [Line Items]
Revenues									33,552.1	42,052.0	66,264.4
Operating revenues									20.6	26.8	19.1
Revenues, Net of Interest Expense									29.7	34.1	28.2
Net Segment Contribution									14.1	17.5	11.6
Segment Income									5.9	10.0	5.2
Assets	116.8				132.5				116.8	132.5	426.9
Other Segment [Member]
Segment Reporting Information [Line Items]
Revenues									113.7	121.3	111.9
Operating revenues									113.7	121.3	111.9
Revenues, Net of Interest Expense									17.9	23.4	16.4
Net Segment Contribution									24.0	25.9	22.9
Segment Income									6.3	5.9	3.5
Assets	1,136.2				1,333.3				1,136.2	1,333.3	1,161.3
Corporate [Member]
Segment Reporting Information [Line Items]
Revenues									(3.1)	7.2	(0.2)
Operating revenues									(3.1)	7.2	(0.2)
Revenues, Net of Interest Expense									(8.9)	4.5	(2.7)
Assets	$ 98.4				$ 115.7				$ 98.4	$ 115.7	$ 98.3