Acquisitions - Gain Capital Holdings Inc Post Acquisition Results and Unaudited Pro Forma Information (Details) - Gain Capital Holdings Inc $ / shares in Units, $ in Millions	12 Months Ended
	Sep. 30, 2020 USD ($) $ / shares
Business Acquisition, Pro Forma Information [Abstract]
Total revenues | $	$ 54,414.1
Net income | $	$ 138.5
Basic earnings per share (in dollars per share) | $ / shares	$ 7.17
Diluted earnings per share (in dollars per share) | $ / shares	$ 7.02