Revenues From Contracts With Clients Commissions and Clearing Fees (Policies)	3 Months Ended
	Dec. 31, 2018
Commissions and Clearing Fees [Abstract]
Contract with Customer, Timing of Satisfaction of Performance Obligation and Payment	<div style="font-family:Times New Roman;font-size:10pt;"><div style="line-height:120%;padding-bottom:8px;text-align:left;font-size:10pt;"><font style="font-family:inherit;font-size:10pt;font-weight:bold;">Commission and Clearing Fees</font></div><div style="line-height:120%;padding-bottom:8px;padding-top:8px;text-align:left;font-size:10pt;"><font style="font-family:inherit;font-size:10pt;">Commission revenue represents sales and brokerage commissions generated by internal brokers, introducing broker-dealers, or registered investment advisors of introducing-broker dealers for their clients’ trading activity in futures, options on futures, OTC derivatives, fixed income securities, equity securities, mutual funds, and annuities. The Company views the selling, distribution, and marketing, or any combination thereof, of mutual funds and insurance and annuity products to clients on the Company’s registered investment advisor (“RIA”) platform as a single performance obligation to the product sponsors. </font></div><div style="line-height:120%;padding-bottom:8px;padding-top:8px;text-align:left;font-size:10pt;"><font style="font-family:inherit;font-size:10pt;">The Company is the principal for commission revenue, as it is responsible for the execution of the clients’ purchases and sales, and maintains relationships with product sponsors for trailing commission. Introducing broker dealers and registered investment advisors assist the Company in performing its obligations. Accordingly, total commission revenues are reported on a gross basis. </font></div><div style="line-height:120%;padding-bottom:8px;padding-top:8px;text-align:left;font-size:10pt;"><font style="font-family:inherit;font-size:10pt;">The Company primarily generates commission revenue on exchange-traded derivatives, OTC derivatives, and securities. Exchange-traded and OTC derivative commissions are recognized at a point in time on the trade date when the client, either directly or through the use of an internal broker or introducing broker, requests the clearance and execution of a trade. Securities commissions are either sale-based commission that are recognized at a point in time on the trade date or trailing commission that are recognized over time as earned. Sales-based securities commission is for a flat fee per security transaction or in certain instances are based on a percentage of an investment product’s current market value at the time of purchase. Trailing commission revenue is generally based on a percentage of the current market value of clients’ investment holdings in trail-eligible assets, and is recognized over the period during which services, such as on-going support, are performed. As trailing commission revenue is based on the market value of clients’ investment holdings in trail-eligible assets, this variable consideration is constrained until the market value of trail-eligible assets is determinable. </font></div><div style="line-height:120%;padding-bottom:8px;padding-top:8px;text-align:left;font-size:10pt;"><font style="font-family:inherit;font-size:10pt;">Clearing fees generally represent transactional based fees charged by the various exchanges and clearing organizations for which the Company or one of its clearing brokers is a member for the privilege of executing and clearing trades through them. Clearing fees are generally passed through to the clients’ accounts and are reported gross as the Company maintains control over the clearing and execution services provided, maintains relationships with the exchanges or clearing brokers, and has ultimate discretion in whether the fees are passed through to the clients and the rates at which they are passed through. As clearing fees are transactional based revenues they are recognized at a point in time on the trade date along with the related commission revenue when the client requests the clearance and execution of a trade. </font></div></div>