Revenue from Contracts with Clients Disaggregation of Revenues Table (Details) - USD ($)	3 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	$ 108,500,000	$ 116,500,000
Revenues	11,245,000,000	6,552,200,000
Consulting, management, and account fees	21,300,000	19,100,000
Transferred at Point in Time [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	84,600,000	94,600,000
Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	23,900,000	21,900,000
UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues	512,300,000	527,000,000
Europe [Member]
Disaggregation of Revenue [Line Items]
Revenues	111,600,000	53,300,000
South America [Member]
Disaggregation of Revenue [Line Items]
Revenues	15,200,000	11,400,000
Middle East and Asia [Domain]
Disaggregation of Revenue [Line Items]
Revenues	10,605,500,000	5,958,700,000
Other (geographic location) [Member]
Disaggregation of Revenue [Line Items]
Revenues	400,000	1,800,000
Commission and Clearing Fees [Member]
Disaggregation of Revenue [Line Items]
Revenues	87,200,000	97,400,000
Sales Based Commissions [Domain]
Disaggregation of Revenue [Line Items]
Sales Commissions and Fees	45,900,000	53,400,000
Sales Based Commissions [Domain] | Exhange-Traded Futures and Options [Member]
Disaggregation of Revenue [Line Items]
Revenues	32,400,000	39,100,000
Sales Based Commissions [Domain] | OTC Derivative Brokerage [Member]
Disaggregation of Revenue [Line Items]
Revenues	5,500,000	8,900,000
Sales Based Commissions [Domain] | Equities and Fixed Income Commissions [Member]
Disaggregation of Revenue [Line Items]
Revenues	4,300,000	1,100,000
Sales Based Commissions [Domain] | Mutual Fund Sales Based Commissions [Member]
Disaggregation of Revenue [Line Items]
Revenues	1,300,000	2,600,000
Sales Based Commissions [Domain] | Variable Annuity [Member]
Disaggregation of Revenue [Line Items]
Revenues	2,100,000	1,500,000
Sales Based Commissions [Domain] | Other Sales Based Commissions [Member]
Disaggregation of Revenue [Line Items]
Revenues	300,000	200,000
Trailing Commissions [Domain]
Disaggregation of Revenue [Line Items]
Revenues	6,800,000	6,900,000
Trailing Commissions [Domain] | Variable Annuity [Member]
Disaggregation of Revenue [Line Items]
Revenues	3,700,000	3,700,000
Trailing Commissions [Domain] | Mutual Fund Trailing Commissions [Member]
Disaggregation of Revenue [Line Items]
Revenues	3,100,000	3,200,000
Interest Income [Member]
Disaggregation of Revenue [Line Items]
Revenues	46,000,000	45,000,000
Clearing Fees [Domain]
Disaggregation of Revenue [Line Items]
Revenues	29,500,000	32,600,000
Trade Conversion Fees [Domain]
Disaggregation of Revenue [Line Items]
Revenues	1,500,000	1,600,000
Other Commissions [Domain]
Disaggregation of Revenue [Line Items]
Revenues	3,500,000	2,900,000
Consulting, management, and account fees [Domain] | Sweep Program Fees [Member]
Disaggregation of Revenue [Line Items]
Revenues	4,000,000	3,800,000
Consulting, management, and account fees [Domain] | Advisory and Consulting Fees [Member]
Disaggregation of Revenue [Line Items]
Revenues	5,600,000	5,000,000
Consulting, management, and account fees [Domain] | Asset Management [Member]
Disaggregation of Revenue [Line Items]
Revenues	7,500,000	6,200,000
Consulting, management, and account fees [Domain] | Underwriting Fees [Member]
Disaggregation of Revenue [Line Items]
Revenues	200,000	300,000
Consulting, management, and account fees [Domain] | Client Account Fees [Member]
Disaggregation of Revenue [Line Items]
Revenues	3,000,000	2,700,000
Consulting, management, and account fees [Domain] | Other Consulting, Management, and Account Fees [Member]
Disaggregation of Revenue [Line Items]
Revenues	1,000,000	1,100,000
Principal or Proprietary Transactions [Member]
Disaggregation of Revenue [Line Items]
Revenues	112,500,000	94,900,000
Sales of physical commodities [Member] | Precious Metals Trading Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	10,658,000,000	5,955,600,000
Sales of physical commodities [Member] | Physical Commodity Origination and Merchandising [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ 320,000,000	$ 340,200,000