Commodity and Other Repurchase Agreements (Details) - USD ($)	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Securities Borrowed, Gross	$ 1,423,200,000	$ 225,500,000
Securities Purchased under Agreements to Resell, Gross	1,474,400,000	870,800,000
Securities sold under agreements to repurchase	2,773,700,000	1,936,700,000
Fair Value of Securities Received as Collateral that Can be Resold or Repledged	3,060,200,000
Fair Value of Securities Received as Collateral that Have Been Resold or Repledged	329,800,000
Financial Instruments Owned and Pledged as Collateral, Amount Eligible to be Repledged by Counterparty	478,800,000
Securities Loaned, Fair Value of Collateral	1,414,000,000.0
Financial Instruments Owned and Pledged as Collateral, Amount Not Eligible to be Repledged by Counterparty	1,228,900,000
Securities Loaned	1,459,900,000	277,900,000
Collateralized Financings	4,233,600,000	2,214,600,000
Securities Purchased under Agreements to Resell, Liability	(49,900,000)	0
Securities purchased under agreements to resell	1,424,500,000	870,800,000
Securities Borrowed, Liability	0	0
Securities Borrowed	1,423,200,000	225,500,000
Securities Sold under Agreements to Repurchase, Gross	2,823,600,000	1,936,700,000
Securities Sold under Agreements to Repurchase, Asset	(49,900,000)	0
Securities Loaned, Gross	1,459,900,000	277,900,000
Securities Loaned, Asset	0	0
Sales Repurchase Agreements	0	1,900,000
Securities Sold under Agreements to Repurchase, Fair Value of Collateral		478,800,000	$ 123,000,000
Maturity Overnight [Member]
Securities sold under agreements to repurchase	1,553,900,000	934,900,000
Securities Loaned	1,459,900,000	277,900,000
Collateralized Financings	3,013,800,000	1,212,800,000
Maturity Less than 30 Days [Member]
Securities sold under agreements to repurchase	565,800,000	661,300,000
Securities Loaned	0	0
Collateralized Financings	565,800,000	661,300,000
Maturity 30 to 90 Days [Member]
Securities sold under agreements to repurchase	654,000,000	340,500,000
Securities Loaned	0	0
Collateralized Financings	654,000,000	340,500,000
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Securities sold under agreements to repurchase	2,208,700,000.0	1,385,400,000.0
Asset-backed Securities [Member]
Securities sold under agreements to repurchase	96,700,000	50,000,000
US Government Debt Securities [Member]
Securities sold under agreements to repurchase	108,800,000	39,600,000
US Government Agencies Debt Securities [Member]
Securities sold under agreements to repurchase	359,500,000	461,700,000
Equity Securities [Member]
Securities Loaned	$ 1,459,900,000	$ 277,900,000