Revenues From Contracts With Clients Disaggregation of Revenues (Details) - USD ($)	3 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Commissions and clearing fees:
Total commission and clearing fees	$ 97.4	$ 85.3
Consulting, management, and account fees	19.1	16.6
Total revenues from contracts with clients	116.5	101.9
Sales of physical commodities	6,295.8	7,714.4
Principal gains, net	94.9	78.3
Interest income	45	24
Total revenues	6,552.2	7,918.6
Transferred at Point in Time [Member]
Commissions and clearing fees:
Total revenues from contracts with clients	94.6	82.2
Transferred over Time [Member]
Commissions and clearing fees:
Total revenues from contracts with clients	21.9	19.7
UNITED STATES
Commissions and clearing fees:
Total revenues	527	324
Europe [Member]
Commissions and clearing fees:
Total revenues	53.3	45.2
South America [Member]
Commissions and clearing fees:
Total revenues	11.4	14.9
Middle East and Asia [Domain]
Commissions and clearing fees:
Total revenues	5,958.7	7,533.6
Other (geographic location) [Member]
Commissions and clearing fees:
Total revenues	1.8	0.9
Precious Metals Trading Revenues [Member]
Commissions and clearing fees:
Sales of physical commodities	5,955.6	7,531.9
Physical Commodity Origination and Merchandising [Member]
Commissions and clearing fees:
Sales of physical commodities	340.2	182.5
Sales Based Commissions [Domain]
Commissions and clearing fees:
Total sales-based commissions	53.4	47.9
Sales Based Commissions [Domain] | Exhange-Traded Futures and Options [Member]
Commissions and clearing fees:
Total commission and clearing fees	39.1	34.6
Sales Based Commissions [Domain] | OTC Derivative Brokerage [Member]
Commissions and clearing fees:
OTC derivative brokerage	8.9	7.5
Sales Based Commissions [Domain] | Equities and Fixed Income Commissions [Member]
Commissions and clearing fees:
Total commission and clearing fees	1.1	1.5
Sales Based Commissions [Domain] | Mutual Fund Sales Based Commissions [Member]
Commissions and clearing fees:
Total commission and clearing fees	2.6	2
Sales Based Commissions [Domain] | Variable Annuity [Member]
Commissions and clearing fees:
Total commission and clearing fees	1.5	1.6
Sales Based Commissions [Domain] | Other Sales Based Commissions [Member]
Commissions and clearing fees:
Total commission and clearing fees	0.2	0.7
Trailing Commissions [Domain]
Commissions and clearing fees:
Total commission and clearing fees	6.9	6.9
Trailing Commissions [Domain] | Mutual Fund Trailing Commissions [Member]
Commissions and clearing fees:
Total commission and clearing fees	3.2	3.3
Trailing Commissions [Domain] | Variable Annuity [Member]
Commissions and clearing fees:
Total commission and clearing fees	3.7	3.6
Clearing Fees [Domain]
Commissions and clearing fees:
Clearing fees	32.6	24.9
Trade Conversion Fees [Domain]
Commissions and clearing fees:
Trade conversion fees	1.6	1.8
Other Commissions [Domain]
Commissions and clearing fees:
Trade conversion fees	2.9	3.8
Consulting, management, and account fees [Domain] | Underwriting Fees [Member]
Commissions and clearing fees:
Underwriting fees	0.3	0.7
Consulting, management, and account fees [Domain] | Asset Management Fees [Member]
Commissions and clearing fees:
Consulting, management, and account fees	6.2	6.1
Consulting, management, and account fees [Domain] | Advisory and Consulting Fees [Member]
Commissions and clearing fees:
Consulting, management, and account fees	5.0	4.6
Consulting, management, and account fees [Domain] | Sweep Program Fees [Member]
Commissions and clearing fees:
Consulting, management, and account fees	3.8	2.1
Consulting, management, and account fees [Domain] | Client Account Fees [Member]
Commissions and clearing fees:
Consulting, management, and account fees	2.7	2.8
Consulting, management, and account fees [Domain] | Other Consulting, Management, and Account Fees [Member]
Commissions and clearing fees:
Consulting, management, and account fees	$ 1.1	$ 0.3