Segment and Geographic Information (Details) - USD ($)	3 Months Ended								12 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information [Line Items]
Total revenues	$ 11,279,600,000	$ 7,873,000,000	$ 7,192,200,000	$ 6,552,200,000	$ 6,078,800,000	$ 7,118,300,000	$ 6,507,000,000	$ 7,918,600,000	$ 32,897,000,000	$ 27,622,700,000	$ 29,423,600,000
Operating revenues	286,900,000	283,400,000	271,100,000	264,700,000	243,200,000	259,800,000	260,200,000	212,600,000	1,106,100,000	975,800,000	784,000,000
Net operating revenues	173,400,000	165,600,000	165,200,000	149,000,000	142,400,000	154,600,000	154,300,000	130,300,000	653,200,000	581,600,000	492,600,000
Net Segment Contribution									482,800,000	432,400,000	387,000,000
Segment Income									305,700,000	261,900,000	169,000,000
Costs not allocated to operating segments									200,200,000	162,400,000	153,800,000
Other gains	100,000	0	5,400,000	0	0	2,000,000	0	0	5,500,000	2,000,000	0
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	34,100,000	21,600,000	$ 30,900,000	$ 24,400,000	20,500,000	$ 32,900,000	$ 29,500,000	$ 18,600,000	111,000,000	101,500,000	15,200,000
Assets	$ 9,936,100,000				7,824,700,000				9,936,100,000	7,824,700,000	6,243,400,000
Commercial Hedging [Member]
Segment Reporting Information [Line Items]
Total revenues									302,400,000	286,700,000	244,600,000
Operating revenues									302,400,000	286,700,000	244,600,000
Net operating revenues									236,500,000	226,400,000	194,300,000
Net Segment Contribution									170,400,000	164,700,000	141,800,000
Segment Income									100,100,000	96,400,000	72,800,000
Assets		2,041,000,000			1,935,700,000					1,935,700,000	1,650,300,000
Global Payments [Member]
Segment Reporting Information [Line Items]
Total revenues									112,800,000	99,200,000	89,200,000
Operating revenues									112,800,000	99,200,000	89,200,000
Net operating revenues									107,000,000	93,500,000	80,600,000
Net Segment Contribution									86,600,000	75,000,000	64,400,000
Segment Income									66,100,000	59,800,000	50,600,000
Assets		278,300,000			206,600,000					206,600,000	199,500,000
Securities Segment [Member]
Segment Reporting Information [Line Items]
Total revenues									295,300,000	196,200,000	151,700,000
Operating revenues									295,300,000	196,200,000	151,700,000
Net operating revenues									131,000,000	94,600,000	94,600,000
Net Segment Contribution									81,900,000	69,100,000	75,600,000
Segment Income									47,400,000	40,800,000	46,600,000
Assets		5,219,100,000			3,058,200,000					3,058,200,000	2,101,700,000
Physical Commodities Segment [Member]
Segment Reporting Information [Line Items]
Total revenues									31,864,700,000	26,703,800,000	28,684,400,000
Operating revenues									73,800,000	56,900,000	44,800,000
Net operating revenues									56,300,000	44,800,000	37,300,000
Net Segment Contribution									40,500,000	31,800,000	27,200,000
Segment Income									38,000,000	16,600,000	(31,400,000)
Assets		357,800,000			413,700,000					413,700,000	339,500,000
Clearing and Execution Services Segment [Member]
Segment Reporting Information [Line Items]
Total revenues									326,100,000	332,400,000	259,800,000
Operating revenues									326,100,000	332,400,000	259,800,000
Net operating revenues									133,200,000	122,600,000	102,200,000
Net Segment Contribution									103,400,000	91,800,000	78,000,000
Segment Income									54,100,000	48,300,000	30,400,000
Assets		1,892,100,000			2,109,900,000					2,109,900,000	1,818,900,000
Corporate [Member]
Segment Reporting Information [Line Items]
Total revenues									20,800,000	23,900,000	4,700,000
Operating revenues									20,800,000	23,900,000	4,700,000
Net operating revenues									$ (10,800,000)	(300,000)	(16,400,000)
Assets		$ 147,800,000			$ 100,600,000					$ 100,600,000	$ 133,500,000