Revenue from Contracts with Clients Disaggregation of Revenues Table (Details) - USD ($)	3 Months Ended		6 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	$ 139,200,000	$ 104,200,000	$ 247,700,000	$ 222,800,000
Revenues	20,366,300,000	7,192,200,000	31,611,300,000	13,744,400,000
Consulting, management, and account fees	22,600,000	19,100,000	43,900,000	38,200,000
Transferred at Point in Time [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	113,700,000	83,000,000	198,300,000	179,700,000
Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	25,500,000	21,200,000	49,400,000	43,100,000
UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues	585,600,000	456,600,000	1,097,900,000	983,600,000
Europe [Member]
Disaggregation of Revenue [Line Items]
Revenues	150,000,000	53,800,000	261,600,000	107,100,000
South America [Member]
Disaggregation of Revenue [Line Items]
Revenues	16,300,000	12,400,000	31,500,000	23,900,000
Middle East and Asia [Domain]
Disaggregation of Revenue [Line Items]
Revenues	19,613,300,000	6,667,300,000	30,218,800,000	12,625,900,000
Other (geographic location) [Member]
Disaggregation of Revenue [Line Items]
Revenues	1,100,000	2,100,000	1,500,000	3,900,000
Commission and Clearing Fees [Member]
Disaggregation of Revenue [Line Items]
Revenues	116,600,000	85,100,000	203,800,000	184,600,000
Sales Based Commissions [Domain]
Disaggregation of Revenue [Line Items]
Sales Commissions and Fees	58,400,000	46,000,000	104,300,000	101,500,000
Sales Based Commissions [Domain] | Exhange-Traded Futures and Options [Member]
Disaggregation of Revenue [Line Items]
Revenues	40,100,000	31,200,000	72,500,000	70,300,000
Sales Based Commissions [Domain] | OTC Derivative Brokerage [Member]
Disaggregation of Revenue [Line Items]
Revenues	6,700,000	7,300,000	12,200,000	16,200,000
Sales Based Commissions [Domain] | Equities and Fixed Income Commissions [Member]
Disaggregation of Revenue [Line Items]
Revenues	7,100,000	3,600,000	11,400,000	6,800,000
Sales Based Commissions [Domain] | Mutual Fund Sales Based Commissions [Member]
Disaggregation of Revenue [Line Items]
Revenues	1,600,000	1,900,000	2,900,000	4,500,000
Sales Based Commissions [Domain] | Variable Annuity [Member]
Disaggregation of Revenue [Line Items]
Revenues	2,400,000	1,500,000	4,500,000	3,000,000
Sales Based Commissions [Domain] | Other Sales Based Commissions [Member]
Disaggregation of Revenue [Line Items]
Revenues	500,000	500,000	800,000	700,000
Trailing Commissions [Domain]
Disaggregation of Revenue [Line Items]
Revenues	7,500,000	6,400,000	14,300,000	13,300,000
Trailing Commissions [Domain] | Variable Annuity [Member]
Disaggregation of Revenue [Line Items]
Revenues	4,000,000	3,400,000	7,700,000	7,100,000
Trailing Commissions [Domain] | Mutual Fund Trailing Commissions [Member]
Disaggregation of Revenue [Line Items]
Revenues	3,500,000	3,000,000	6,600,000	6,200,000
Interest Income [Member]
Disaggregation of Revenue [Line Items]
Revenues	41,700,000	48,200,000	87,700,000	93,200,000
Clearing Fees [Domain]
Disaggregation of Revenue [Line Items]
Revenues	41,600,000	26,800,000	71,100,000	59,400,000
Trade Conversion Fees [Domain]
Disaggregation of Revenue [Line Items]
Revenues	4,200,000	3,300,000	5,700,000	4,900,000
Other Commissions [Domain]
Disaggregation of Revenue [Line Items]
Revenues	4,900,000	2,600,000	8,400,000	5,500,000
Consulting, management, and account fees [Domain] | Sweep Program Fees [Member]
Disaggregation of Revenue [Line Items]
Revenues	3,600,000	4,100,000	7,600,000	7,900,000
Consulting, management, and account fees [Domain] | Advisory and Consulting Fees [Member]
Disaggregation of Revenue [Line Items]
Revenues	5,700,000	4,700,000	11,300,000	9,700,000
Consulting, management, and account fees [Domain] | Asset Management [Member]
Disaggregation of Revenue [Line Items]
Revenues	8,700,000	6,000,000	16,200,000	12,200,000
Consulting, management, and account fees [Domain] | Underwriting Fees [Member]
Disaggregation of Revenue [Line Items]
Revenues	200,000	200,000	400,000	500,000
Consulting, management, and account fees [Domain] | Client Account Fees [Member]
Disaggregation of Revenue [Line Items]
Revenues	3,100,000	2,500,000	6,100,000	5,200,000
Consulting, management, and account fees [Domain] | Other Consulting, Management, and Account Fees [Member]
Disaggregation of Revenue [Line Items]
Revenues	1,300,000	1,600,000	2,300,000	2,700,000
Principal or Proprietary Transactions [Member]
Disaggregation of Revenue [Line Items]
Revenues	168,500,000	110,300,000	281,000,000	203,100,000
Sales of physical commodities [Member] | Precious Metals Trading Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	19,694,700,000	6,660,900,000	30,352,700,000	12,616,400,000
Sales of physical commodities [Member] | Physical Commodity Origination and Merchandising [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ 322,200,000	$ 268,600,000	$ 642,200,000	$ 608,900,000