Fair Value Measurements - Carrying Amounts and Estimated Fair Values of Financial Instruments Not Measured at Fair Value (Details) - USD ($) $ in Millions	Sep. 30, 2026	Dec. 31, 2025
Carrying Amount
Assets:
Cash and cash equivalents	$ 1,731.1	$ 1,387.3
Deposits with banks	106.8	78.5
Notes receivable, net	66.7	35.7
Secured financings receivable	930.3	986.1
Liabilities:
Secured financings payable	807.5	906.5
Notes and contracts payable	1,546.8	1,545.4
Estimated Fair Value
Assets:
Cash and cash equivalents	1,731.1	1,387.3
Deposits with banks	106.5	78.2
Notes receivable, net	68.1	36.1
Secured financings receivable	930.3	986.1
Liabilities:
Secured financings payable	807.5	906.5
Notes and contracts payable	1,392.4	1,459.9
Estimated Fair Value \| Level 1
Assets:
Cash and cash equivalents	1,731.1	1,387.3
Deposits with banks	26.0	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	80.5	68.3
Notes receivable, net	0.0	0.0
Secured financings receivable	930.3	986.1
Liabilities:
Secured financings payable	807.5	906.5
Notes and contracts payable	1,384.7	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	68.1	36.1
Secured financings receivable	0.0	0.0
Liabilities:
Secured financings payable	0.0	0.0
Notes and contracts payable	$ 7.7	$ 7.8