Revenue from Contracts with Clients Disaggregation of Revenues Table (Details) - USD ($)	3 Months Ended		9 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	$ 115,500,000	$ 118,200,000	$ 363,200,000	$ 341,000,000.0
Revenues	8,243,400,000	7,873,000,000.0	39,854,700,000	21,617,400,000
Commission and clearing fees	96,100,000	97,500,000	299,900,000	282,100,000
Consulting, management, and account fees	19,400,000	20,700,000	63,300,000	58,900,000
Transferred at Point in Time [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	95,100,000	95,000,000.0	293,400,000	274,700,000
Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	20,400,000	23,200,000	69,800,000	66,300,000
UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues	554,200,000	474,800,000	1,652,100,000	1,458,400,000
Europe [Member]
Disaggregation of Revenue [Line Items]
Revenues	114,100,000	77,800,000	375,700,000	184,900,000
South America [Member]
Disaggregation of Revenue [Line Items]
Revenues	14,600,000	15,300,000	46,100,000	39,200,000
Middle East and Asia [Domain]
Disaggregation of Revenue [Line Items]
Revenues	7,558,500,000	7,303,800,000	37,777,300,000	19,929,700,000
Other (geographic location) [Member]
Disaggregation of Revenue [Line Items]
Revenues	2,000,000.0	1,300,000	3,500,000	5,200,000
Interest Income [Member]
Disaggregation of Revenue [Line Items]
Revenues	22,400,000	53,200,000	110,100,000	146,400,000
Brokerage commissions revenue [Domain] | Exhange-Traded Futures and Options [Member]
Disaggregation of Revenue [Line Items]
Revenues	36,400,000	40,400,000	108,900,000	110,700,000
Brokerage commissions revenue [Domain] | OTC Derivative Brokerage [Member]
Disaggregation of Revenue [Line Items]
Revenues	4,000,000.0	7,700,000	16,200,000	23,900,000
Brokerage commissions revenue [Domain] | Equities and Fixed Income Commissions [Member]
Disaggregation of Revenue [Line Items]
Revenues	5,200,000	4,300,000	16,600,000	11,100,000
Brokerage commissions revenue [Domain] | Mutual Fund Sales Based Commissions [Member]
Disaggregation of Revenue [Line Items]
Revenues	1,100,000	1,600,000	4,000,000.0	6,100,000
Brokerage commissions revenue [Domain] | insurance and annuity products [Domain] [Domain]
Disaggregation of Revenue [Line Items]
Revenues	1,700,000	2,100,000	6,200,000	5,100,000
Brokerage commissions revenue [Domain] | Other Commissions [Domain]
Disaggregation of Revenue [Line Items]
Revenues	5,600,000	2,800,000	14,000,000.0	8,300,000
Brokerage commissions revenue [Domain] | Other Sales Based Commissions [Member]
Disaggregation of Revenue [Line Items]
Revenues	300,000	300,000	1,100,000	1,000,000.0
Brokerage commissions revenue [Domain] | Sales Based Commissions [Domain]
Disaggregation of Revenue [Line Items]
Sales Commissions and Fees	48,700,000	56,400,000	153,000,000.0	157,900,000
Brokerage commissions revenue [Domain] | Mutual Fund Trailing Commissions [Member]
Disaggregation of Revenue [Line Items]
Revenues	3,100,000	3,100,000	9,700,000	9,300,000
Brokerage commissions revenue [Domain] | Insurance and annuity product trailing commissions [Domain]
Disaggregation of Revenue [Line Items]
Revenues	3,600,000	3,600,000	11,300,000	10,700,000
Brokerage commissions revenue [Domain] | Trade Conversion Fees [Domain]
Disaggregation of Revenue [Line Items]
Revenues	1,800,000	1,300,000	7,500,000	6,200,000
Brokerage commissions revenue [Domain] | Clearing Fees [Domain]
Disaggregation of Revenue [Line Items]
Revenues	33,300,000	30,300,000	104,400,000	89,700,000
Brokerage commissions revenue [Domain] | Trailing Commissions [Domain]
Disaggregation of Revenue [Line Items]
Revenues	6,700,000	6,700,000	21,000,000.0	20,000,000.0
Consulting, management, and account fees [Domain] | Sweep Program Fees [Member]
Disaggregation of Revenue [Line Items]
Revenues	1,000,000.0	4,200,000	8,600,000	12,100,000
Consulting, management, and account fees [Domain] | Advisory and Consulting Fees [Member]
Disaggregation of Revenue [Line Items]
Revenues	5,700,000	5,100,000	17,000,000.0	14,800,000
Consulting, management, and account fees [Domain] | Asset Management [Member]
Disaggregation of Revenue [Line Items]
Revenues	7,000,000.0	7,200,000	23,200,000	19,400,000
Consulting, management, and account fees [Domain] | Underwriting Fees [Member]
Disaggregation of Revenue [Line Items]
Revenues	0	200,000	400,000	700,000
Consulting, management, and account fees [Domain] | Client Account Fees [Member]
Disaggregation of Revenue [Line Items]
Revenues	2,800,000	2,600,000	8,900,000	7,800,000
Consulting, management, and account fees [Domain] | Other Consulting, Management, and Account Fees [Member]
Disaggregation of Revenue [Line Items]
Revenues	2,900,000	1,400,000	5,200,000	4,100,000
Principal or Proprietary Transactions [Member]
Disaggregation of Revenue [Line Items]
Revenues	161,000,000.0	102,300,000	442,000,000.0	305,400,000
Sales of physical commodities [Member] | Precious Metals Trading Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	7,621,200,000	7,327,800,000	37,973,900,000	19,944,200,000
Sales of physical commodities [Member] | Physical Commodity Origination and Merchandising [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ 323,300,000	$ 271,500,000	$ 965,500,000	$ 880,400,000