Revenue from Contracts with Clients Disaggregation of Revenues Table (Details) - USD ($)	3 Months Ended		6 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018
Disaggregation of Revenue [Line Items]
Commission and clearing fees	$ 85,100,000	$ 106,400,000	$ 184,600,000	$ 192,900,000
Underwriting Income (Loss)	200,000	700,000	500,000	1,300,000
Asset Management Fees	6,000,000	6,400,000	12,200,000	12,500,000
Investment Advisory Fees	4,700,000	4,800,000	9,700,000	9,400,000
Consulting,management, and account fees	19,100,000.0	18,400,000.0	38,200,000.0	35,000,000.0
Revenue from Contract with Customer, Including Assessed Tax	104,200,000	124,800,000	222,800,000	227,900,000
Sales of physical commodities	6,929,500,000	6,255,800,000	13,225,300,000	13,970,200,000
Principal gains, net	110,300,000	98,500,000	203,100,000	175,600,000
Interest income	48,200,000	27,900,000	93,200,000	51,900,000
Revenues	7,192,200,000	6,507,000,000	13,744,400,000	14,425,600,000
Transferred at Point in Time [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	83,000,000	103,900,000	179,700,000	187,200,000
Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	21,200,000	20,900,000	43,100,000	40,700,000
UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues	456,600,000	404,100,000	983,600,000	728,000,000
Europe [Member]
Disaggregation of Revenue [Line Items]
Revenues	53,800,000	47,800,000	107,100,000	93,000,000
South America [Member]
Disaggregation of Revenue [Line Items]
Revenues	12,400,000	16,700,000	23,900,000	31,700,000
Middle East and Asia [Domain]
Disaggregation of Revenue [Line Items]
Revenues	6,667,300,000	6,037,400,000	12,625,900,000	13,571,000,000
Other (geographic location) [Member]
Disaggregation of Revenue [Line Items]
Revenues	2,100,000	1,000,000	3,900,000	1,900,000
Precious Metals Trading Revenues [Member]
Disaggregation of Revenue [Line Items]
Sales of physical commodities	6,660,900,000	6,031,600,000	12,616,400,000	13,563,500,000
Physical Commodity Origination and Merchandising [Member]
Disaggregation of Revenue [Line Items]
Sales of physical commodities	268,600,000	224,200,000	608,900,000	406,700,000
Sales Based Commissions [Domain]
Disaggregation of Revenue [Line Items]
Sales Commissions and Fees	46,000,000	58,700,000	101,500,000	107,700,000
Sales Based Commissions [Domain] | Exhange-Traded Futures and Options [Member]
Disaggregation of Revenue [Line Items]
Commission and clearing fees	31,200,000	44,000,000	70,300,000	78,600,000
Sales Based Commissions [Domain] | OTC Derivative Brokerage [Member]
Disaggregation of Revenue [Line Items]
Brokerage Commissions Revenue	7,300,000	8,300,000	16,200,000	15,800,000
Sales Based Commissions [Domain] | Equities and Fixed Income Commissions [Member]
Disaggregation of Revenue [Line Items]
Commission and clearing fees	3,600,000	2,800,000	6,800,000	5,600,000
Sales Based Commissions [Domain] | Mutual Fund Sales Based Commissions [Member]
Disaggregation of Revenue [Line Items]
Commission and clearing fees	1,900,000	2,100,000	4,500,000	4,100,000
Sales Based Commissions [Domain] | Variable Annuity [Member]
Disaggregation of Revenue [Line Items]
Commission and clearing fees	1,500,000	1,500,000	3,000,000	3,000,000
Sales Based Commissions [Domain] | Other Sales Based Commissions [Member]
Disaggregation of Revenue [Line Items]
Commission and clearing fees	500,000	0	700,000	600,000
Trailing Commissions [Domain]
Disaggregation of Revenue [Line Items]
Commission and clearing fees	6,400,000	7,000,000	13,300,000	13,900,000
Trailing Commissions [Domain] | Variable Annuity [Member]
Disaggregation of Revenue [Line Items]
Commission and clearing fees	3,400,000	3,700,000	7,100,000	7,300,000
Trailing Commissions [Domain] | Mutual Fund Trailing Commissions [Member]
Disaggregation of Revenue [Line Items]
Commission and clearing fees	3,000,000	3,300,000	6,200,000	6,600,000
Clearing Fees [Domain]
Disaggregation of Revenue [Line Items]
Clearing Fees Revenue	26,800,000	34,500,000	59,400,000	59,500,000
Trade Conversion Fees [Domain]
Disaggregation of Revenue [Line Items]
Fees and Commissions, Other	3,300,000	2,000,000	4,900,000	3,800,000
Other Commissions [Domain]
Disaggregation of Revenue [Line Items]
Fees and Commissions, Other	2,600,000	4,200,000	5,500,000	8,000,000
Consulting, management, and account fees [Domain] | Sweep Program Fees [Member]
Disaggregation of Revenue [Line Items]
Consulting,management, and account fees	4,100,000.0	2,700,000.0	7,900,000.0	4,900,000.0
Consulting, management, and account fees [Domain] | Client Account Fees [Member]
Disaggregation of Revenue [Line Items]
Consulting,management, and account fees	2,500,000.0	2,900,000.0	5,200,000.0	5,700,000.0
Consulting, management, and account fees [Domain] | Other Consulting, Management, and Account Fees [Member]
Disaggregation of Revenue [Line Items]
Consulting,management, and account fees	$ 1,600,000.0	$ 900,000.0	$ 2,700,000.0	$ 1,200,000.0