Condensed Consolidated Cash Flows Statements Reconciliation of Cash, Segregated Cash, Cash Equivalents, and Segregated Cash Equivalents - USD ($) $ in Millions	Jun. 30, 2020	Jun. 30, 2019	Mar. 31, 2019
Reconciliation of Cash, Segregated Cash, Cash Equivalents, and Segregated Cash Equivalents [Abstract]
Cash and cash equivalents	$ 465.2	$ 345.0
Restricted cash held in escrow	350.0	0.0
Cash segregated under federal and other regulations(1)	1,135.4	721.0
Securities and other assets segregated, fair value	0.0	299.6
Cash segregated and deposited with or pledged to exchange-clearing organizations and other futures commissions merchants	1,218.7	1,223.8
Securities segregated and pledged to exchange-clearing organizations(2)	929.5	39.2
Cash, segregated cash, cash equivalents, and segregated cash equivalents at end of period	4,098.8	$ 2,628.6
Non-cash equivalent segregated assets	9.8		$ 58.8
Non-segregated cash and other non-cash equivalent assets included within deposits and receivables from broker-dealers, clearing organizations, and counterparties	$ 1,248.5		$ 1,398.6