Fair Value Measurements - Carrying Amounts and Estimated Fair Values of Financial Instruments Not Measured at Fair Value (Details) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Assets:
Notes receivable, net	$ 34.4	$ 22.4
Carrying Amount
Assets:
Cash and cash equivalents	1,718.1	3,605.3
Deposits with banks	85.4	55.8
Notes receivable, net	34.4	22.4
Secured financings receivable	690.0	636.5
Liabilities:
Secured financings payable	643.8	553.3
Notes and contracts payable	1,546.6	1,393.9
Estimated Fair Value
Assets:
Cash and cash equivalents	1,718.1	3,605.3
Deposits with banks	85.3	55.6
Notes receivable, net	34.6	23.2
Secured financings receivable	690.0	636.5
Liabilities:
Secured financings payable	643.8	553.3
Notes and contracts payable	1,399.4	1,219.6
Estimated Fair Value | Level 1
Assets:
Cash and cash equivalents	1,718.1	3,605.3
Deposits with banks	20.7	4.0
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	64.6	51.6
Notes receivable, net	0.0	0.0
Secured financings receivable	690.0	636.5
Liabilities:
Secured financings payable	643.8	553.3
Notes and contracts payable	1,388.4	1,215.4
Estimated Fair Value | Level 3
Assets:
Cash and cash equivalents	0.0	0.0
Deposits with banks	0.0	0.0
Notes receivable, net	34.6	23.2
Secured financings receivable	0.0	0.0
Liabilities:
Secured financings payable	0.0	0.0
Notes and contracts payable	$ 11.0	$ 4.2