Condensed Consolidated Cash Flows Statements Reconciliation of Cash, Segregated Cash, Cash Equivalents, and Segregated Cash Equivalents - USD ($) $ in Millions	Mar. 31, 2020	Mar. 31, 2019
Reconciliation of Cash, Segregated Cash, Cash Equivalents, and Segregated Cash Equivalents [Abstract]
Cash and cash equivalents	$ 519.5	$ 274.4
Cash segregated under federal and other regulations(1)	1,168.0	723.3
Cash segregated and deposited with or pledged to exchange-clearing organizations and other futures commissions merchants	1,896.7	993.9
Securities segregated and pledged to exchange-clearing organizations(2)	106.2	118.8
Cash, segregated cash, cash equivalents, and segregated cash equivalents at end of period	3,690.4	2,110.4
Non-cash equivalent segregated assets	8.1	235.7
Non-segregated cash and other non-cash equivalent assets included within deposits and receivables from broker-dealers, clearing organizations, and counterparties	$ 1,356.6	$ 1,154.1