Condensed Consolidated Balance Sheets Parentheticals - USD ($) $ in Millions	Mar. 31, 2018	Sep. 30, 2017
Balance Sheet Parentheticals [Abstract]
Securities and other assets segregated, fair value	$ 675.4	$ 54.5
Dep and rec -b/d - fair value	650.4	204.7
Physical commodities inventory at fair value	111.1	73.2
Collateral that can be sold or repledged	17.3	19.4
Accounts pay and other accrued - fair value	0.0	1.0
Payables to b/d - fair value	$ 7.1	$ 4.8
Preferred stock - par value	$ 0.01	$ 0.01
Preferred stock - authorized	1,000,000	1,000,000
Preferred stock - issued	0	0
Preferred stock - outstanding	0	0
Common stock - par value	$ 0.01	$ 0.01
Common stock - authorized	30,000,000	30,000,000
Common stock - issued	21,012,531	20,855,243
Common stock - outstanding	18,890,574	18,733,286
Treasury stock - shares	2,121,957	2,121,957