Consolidated Balance Sheets - USD ($) $ in Millions	Sep. 30, 2016	Sep. 30, 2015
Statement of Financial Position [Abstract]
Cash and cash equivalents	$ 316.2	$ 268.1
Cash, securities and other assets segregated under federal and other regulations (including $618.8 and $515.5 at fair value at September 30, 2016 and September 30, 2015, respectively)	1,136.3	756.9
Securities purchased under agreements to resell	609.6	325.3
Deposits with and receivables from:
Exchange-clearing organizations (including $868.5 and $1,009.4 at fair value at September 30, 2016 and September 30, 2016, respectively)	1,524.4	1,533.5
Broker-dealers, clearing organizations and counterparties (including $(15.2) and $(52.9) at fair value at September 30, 2016 and September 30, 2015, respectively)	237.0	277.6
Receivables from customers, net	194.5	217.3
Notes receivable, net	18.9	78.4
Income taxes receivable	1.1	10.6
Financial instruments owned, at fair value (includes securities pledged as collateral that can be sold or repledged of $47.2 and $170.7 at September 30, 2016 and September 30, 2015, respectively)	1,606.1	1,421.9
Physical commodities inventory (including $71.2 and $15.2 at fair value at September 30, 2016 and September 30, 2015, respectively)	123.8	32.8
Deferred income taxes, net	34.5	28.2
Property and equipment, net	29.4	19.7
Goodwill and intangible assets, net	56.6	58.1
Other assets	62.9	41.6
Total assets	5,951.3	5,070.0
Liabilities:
Accounts payable and other accrued liabilities (including $0.8 and $3.3 at fair value at September 30, 2016 and September 30, 2015)	161.3	144.8
Payable to:
Customers	2,854.2	2,593.5
Broker-dealers, clearing organizations and counterparties (including $3.5 and $1.6 at fair value at September 30, 2016 and September 30, 2015, respectively)	260.1	262.9
Lenders under loans	182.8	41.6
Senior unsecured notes	45.5	45.5
Income taxes payable	7.1	9.0
Securities sold under agreements to repurchase	1,167.1	1,007.3
Financial instruments sold, not yet purchased, at fair value	839.4	568.3
Total liabilities	5,517.5	4,672.9
Commitments and contingencies (Note 11)
Stockholders’ equity:
Preferred stock, $0.01 par value. Authorized 1,000,000 shares; no shares issued or outstanding	0.0	0.0
Common stock, $0.01 par value. Authorized 30,000,000 shares; 20,557,175 issued and 18,435,218 outstanding at September 30, 2016 and 20,184,556 issued and 18,812,803 outstanding at September 30, 2015	0.2	0.2
Common stock in treasury, at cost - 2,121,957 shares at September 30, 2016 and 1,371,753 shares at September 30, 2015	(46.3)	(26.8)
Additional paid-in capital	249.4	240.8
Retained earnings	255.1	200.4
Accumulated other comprehensive loss, net	(24.6)	(17.5)
Total stockholders' equity	433.8	397.1
Total liabilities and stockholders' equity	$ 5,951.3	$ 5,070.0