Commodity and Other Repurchase Agreements Commodity and Other Repurchase Agreements (Details) - USD ($)	Jun. 30, 2019	Sep. 30, 2018
Financial Instruments Owned and Pledged as Collateral [Line Items]
Financing Receivable, Gross	$ 1,300,000	$ 1,900,000
Fair Value of Securities Received as Collateral that Can be Resold or Repledged	2,904,800,000	1,294,800,000
Financial Instruments Owned and Pledged as Collateral, Amount Eligible to be Repledged by Counterparty	370,900,000	123,000,000
Securities Loaned, Fair Value of Collateral	1,213,300,000.0	267,900,000.0
Securities Sold under Agreements to Repurchase, Fair Value of Collateral	285,500,000	123,000,000
Trading Securities Pledged as Collateral	1,450,800,000	1,481,100,000
Pledged Financial Instruments, Not Separately Reported, Securities for Repurchase Agreements	1,134,500,000	369,800,000
Fair Value of Securities Received as Collateral that Have Been Resold or Repledged	$ 427,100,000	$ 473,900,000