Fair Value Measurements - Carrying Amounts and Estimated Fair Values of Financial Instruments Not Measured at Fair Value (Details) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Carrying Amount
Assets:
Cash and cash equivalents	$ 2,953.4	$ 3,605.3
Deposits with banks	55.4	55.8
Notes receivable, net	28.0	22.4
Secured financings receivable	984.0	636.5
Liabilities:
Secured financings payable	878.2	553.3
Notes and contracts payable	1,847.7	1,393.9
Estimated Fair Value
Assets:
Cash and cash equivalents	2,953.4	3,605.3
Deposits with banks	55.4	55.6
Notes receivable, net	29.1	23.2
Secured financings receivable	984.0	636.5
Liabilities:
Secured financings payable	878.8	553.3
Notes and contracts payable	1,732.8	1,219.6
Estimated Fair Value | Level 1
Assets:
Cash and cash equivalents	2,953.4	3,605.3
Deposits with banks	2.9	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	52.5	51.6
Notes receivable, net	0.0	0.0
Secured financings receivable	984.0	636.5
Liabilities:
Secured financings payable	878.8	553.3
Notes and contracts payable	1,720.2	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	29.1	23.2
Secured financings receivable	0.0	0.0
Liabilities:
Secured financings payable	0.0	0.0
Notes and contracts payable	$ 12.6	$ 4.2