Fair Value Measurements - Carrying Amounts and Estimated Fair Values of Financial Instruments Not Measured at Fair Value (Details) - USD ($) $ in Millions	Jun. 30, 2026	Dec. 31, 2025
Carrying Amount
Assets:
Cash and cash equivalents	$ 2,624.8	$ 1,387.3
Deposits with banks	103.3	78.5
Notes receivable, net	43.4	35.7
Secured financings receivable	1,152.2	986.1
Liabilities:
Secured financings payable	1,036.5	906.5
Notes and contracts payable	1,546.5	1,545.4
Estimated Fair Value
Assets:
Cash and cash equivalents	2,624.8	1,387.3
Deposits with banks	103.1	78.2
Notes receivable, net	43.5	36.1
Secured financings receivable	1,152.2	986.1
Liabilities:
Secured financings payable	1,036.5	906.5
Notes and contracts payable	1,450.8	1,459.9
Estimated Fair Value \| Level 1
Assets:
Cash and cash equivalents	2,624.8	1,387.3
Deposits with banks	30.8	9.9
Notes receivable, net	0.0	0.0
Secured financings receivable	0.0	0.0
Liabilities:
Secured financings payable	0.0	0.0
Notes and contracts payable	0.0	0.0
Estimated Fair Value \| Level 2
Assets:
Cash and cash equivalents	0.0	0.0
Deposits with banks	72.3	68.3
Notes receivable, net	0.0	0.0
Secured financings receivable	1,152.2	986.1
Liabilities:
Secured financings payable	1,036.5	906.5
Notes and contracts payable	1,442.9	1,452.1
Estimated Fair Value \| Level 3
Assets:
Cash and cash equivalents	0.0	0.0
Deposits with banks	0.0	0.0
Notes receivable, net	43.5	36.1
Secured financings receivable	0.0	0.0
Liabilities:
Secured financings payable	0.0	0.0
Notes and contracts payable	$ 7.9	$ 7.8