Revenue from Contracts with Clients Disaggregation of Revenues Table (Details) - USD ($)	3 Months Ended		9 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	$ 118,200,000	$ 123,500,000	$ 341,000,000	$ 351,400,000
Revenues	7,873,000,000	7,118,300,000	21,617,400,000	21,543,900,000
Transferred at Point in Time [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	95,000,000	102,000,000	274,700,000	289,200,000
Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	23,200,000	21,500,000	66,300,000	62,200,000
UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues	474,800,000	431,900,000	1,458,400,000	1,159,900,000
Europe [Member]
Disaggregation of Revenue [Line Items]
Revenues	77,800,000	48,500,000	184,900,000	141,500,000
South America [Member]
Disaggregation of Revenue [Line Items]
Revenues	15,300,000	16,700,000	39,200,000	48,400,000
Middle East and Asia [Domain]
Disaggregation of Revenue [Line Items]
Revenues	7,303,800,000	6,620,000,000	19,929,700,000	20,191,000,000
Other (geographic location) [Member]
Disaggregation of Revenue [Line Items]
Revenues	1,300,000	1,200,000	5,200,000	3,100,000
Commission and Clearing Fees [Member]
Disaggregation of Revenue [Line Items]
Revenues	97,500,000	105,100,000	282,100,000	298,000,000
Interest Income [Member]
Disaggregation of Revenue [Line Items]
Revenues	53,200,000	33,700,000	146,400,000	85,600,000
Sales Based Commissions [Domain]
Disaggregation of Revenue [Line Items]
Sales Commissions and Fees	56,400,000	58,600,000	157,900,000	166,300,000
Sales Based Commissions [Domain] | Exhange-Traded Futures and Options [Member]
Disaggregation of Revenue [Line Items]
Revenues	40,400,000	45,000,000	110,700,000	123,600,000
Sales Based Commissions [Domain] | OTC Derivative Brokerage [Member]
Disaggregation of Revenue [Line Items]
Revenues	7,700,000	7,300,000	23,900,000	23,100,000
Sales Based Commissions [Domain] | Equities and Fixed Income Commissions [Member]
Disaggregation of Revenue [Line Items]
Revenues	4,300,000	2,700,000	11,100,000	8,300,000
Sales Based Commissions [Domain] | Mutual Fund Sales Based Commissions [Member]
Disaggregation of Revenue [Line Items]
Revenues	1,600,000	1,500,000	6,100,000	5,600,000
Sales Based Commissions [Domain] | Variable Annuity [Member]
Disaggregation of Revenue [Line Items]
Revenues	2,100,000	1,500,000	5,100,000	4,500,000
Sales Based Commissions [Domain] | Other Sales Based Commissions [Member]
Disaggregation of Revenue [Line Items]
Revenues	300,000	600,000	1,000,000	1,200,000
Trailing Commissions [Domain]
Disaggregation of Revenue [Line Items]
Revenues	6,700,000	6,900,000	20,000,000	20,800,000
Trailing Commissions [Domain] | Variable Annuity [Member]
Disaggregation of Revenue [Line Items]
Revenues	3,600,000	3,700,000	10,700,000	11,000,000
Trailing Commissions [Domain] | Mutual Fund Trailing Commissions [Member]
Disaggregation of Revenue [Line Items]
Revenues	3,100,000	3,200,000	9,300,000	9,800,000
Clearing Fees [Domain]
Disaggregation of Revenue [Line Items]
Revenues	30,300,000	34,200,000	89,700,000	93,700,000
Trade Conversion Fees [Domain]
Disaggregation of Revenue [Line Items]
Revenues	1,300,000	1,600,000	6,200,000	5,400,000
Other Commissions [Domain]
Disaggregation of Revenue [Line Items]
Revenues	2,800,000	3,800,000	8,300,000	11,800,000
Consulting, management, and account fees [Domain] | Sweep Program Fees [Member]
Disaggregation of Revenue [Line Items]
Revenues	4,200,000.0	3,300,000.0	12,100,000.0	8,200,000.0
Consulting, management, and account fees [Domain] | Advisory and Consulting Fees [Member]
Disaggregation of Revenue [Line Items]
Revenues	5,100,000	4,900,000	14,800,000	14,300,000
Consulting, management, and account fees [Domain] | Asset Management [Member]
Disaggregation of Revenue [Line Items]
Revenues	7,200,000	6,400,000	19,400,000	18,900,000
Consulting, management, and account fees [Domain] | Underwriting Fees [Member]
Disaggregation of Revenue [Line Items]
Revenues	200,000	200,000	700,000	1,500,000
Consulting, management, and account fees [Domain] | Client Account Fees [Member]
Disaggregation of Revenue [Line Items]
Revenues	2,600,000.0	2,900,000.0	7,800,000.0	8,600,000.0
Consulting, management, and account fees [Domain] | Other Consulting, Management, and Account Fees [Member]
Disaggregation of Revenue [Line Items]
Revenues	1,400,000.0	700,000.0	4,100,000.0	1,900,000.0
Other Income [Member]
Disaggregation of Revenue [Line Items]
Revenues	20,700,000.0	18,400,000.0	58,900,000.0	53,400,000.0
Principal or Proprietary Transactions [Member]
Disaggregation of Revenue [Line Items]
Revenues	102,300,000	94,900,000	305,400,000	270,500,000
Sales [Member]
Disaggregation of Revenue [Line Items]
Revenues	7,599,300,000	6,866,200,000	20,824,600,000	20,836,400,000
Sales [Member] | Precious Metals Trading Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	7,327,800,000	6,613,000,000	19,944,200,000	20,176,500,000
Sales [Member] | Physical Commodity Origination and Merchandising [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ 271,500,000	$ 253,200,000	$ 880,400,000	$ 659,900,000