Fair Value Measurements - Carrying Amounts and Estimated Fair Values of Financial Instruments Not Measured at Fair Value (Details) - USD ($) $ in Millions	Jun. 30, 2025	Dec. 31, 2024
Carrying Amount
Assets:
Cash and cash equivalents	$ 2,031.2	$ 1,718.1
Deposits with banks	82.7	85.4
Notes receivable, net	41.1	34.4
Secured financings receivable	975.1	690.0
Liabilities:
Secured financings payable	883.8	643.8
Notes and contracts payable	1,546.8	1,546.6
Estimated Fair Value
Assets:
Cash and cash equivalents	2,031.2	1,718.1
Deposits with banks	82.5	85.3
Notes receivable, net	41.9	34.6
Secured financings receivable	975.1	690.0
Liabilities:
Secured financings payable	883.8	643.8
Notes and contracts payable	1,436.7	1,399.4
Estimated Fair Value | Level 1
Assets:
Cash and cash equivalents	2,031.2	1,718.1
Deposits with banks	13.8	20.7
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	68.7	64.6
Notes receivable, net	0.0	0.0
Secured financings receivable	975.1	690.0
Liabilities:
Secured financings payable	883.8	643.8
Notes and contracts payable	1,426.4	1,388.4
Estimated Fair Value | Level 3
Assets:
Cash and cash equivalents	0.0	0.0
Deposits with banks	0.0	0.0
Notes receivable, net	41.9	34.6
Secured financings receivable	0.0	0.0
Liabilities:
Secured financings payable	0.0	0.0
Notes and contracts payable	$ 10.3	$ 11.0