Acquisitions - Gain Capital Holdings Inc Post Acquisition Results and Unaudited Pro Forma Information (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Business Acquisition [Line Items]
Total revenues	$ 14,284.9	$ 8,243.4	$ 20,366.3	$ 11,245.0	$ 11,279.6	$ 7,873.0	$ 7,192.2	$ 6,552.2	$ 54,139.6	$ 32,897.0	$ 27,622.7
Net income									169.6	85.1	$ 55.5
Gain Capital Holdings Inc [Member]
Business Acquisition [Line Items]
Total revenues									49.0
Net income									1.8
Business Acquisition, Pro Forma Information [Abstract]
Total revenues									54,414.1	33,160.0
Net income									$ 138.5	$ 38.3
Basic earnings per share (in dollars per share)									$ 7.17	$ 2.01
Diluted earnings per share (in dollars per share)									$ 7.02	$ 1.97