Fair Value Measurements - Carrying Amounts and Estimated Fair Values of Financial Instruments Not Measured at Fair Value (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Carrying Amount
Assets:
Cash and cash equivalents	$ 1,506.4	$ 3,605.3
Deposits with banks	77.1	55.8
Notes receivable, net	23.8	22.4
Secured financings receivable	831.4	636.5
Liabilities:
Secured financings payable	688.8	553.3
Notes and contracts payable	1,396.0	1,393.9
Estimated Fair Value
Assets:
Cash and cash equivalents	1,506.4	3,605.3
Deposits with banks	77.0	55.6
Notes receivable, net	24.6	23.2
Secured financings receivable	831.4	636.5
Liabilities:
Secured financings payable	688.8	553.3
Notes and contracts payable	1,226.3	1,219.6
Estimated Fair Value | Level 1
Assets:
Cash and cash equivalents	1,506.4	3,605.3
Deposits with banks	20.1	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	56.9	51.6
Notes receivable, net	0.0	0.0
Secured financings receivable	831.4	636.5
Liabilities:
Secured financings payable	688.8	553.3
Notes and contracts payable	1,220.5	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	24.6	23.2
Secured financings receivable	0.0	0.0
Liabilities:
Secured financings payable	0.0	0.0
Notes and contracts payable	$ 5.8	$ 4.2