Fair Value Measurements - Carrying Amounts and Estimated Fair Values of Financial Instruments Not Measured at Fair Value (Details) - USD ($) $ in Millions	Jun. 30, 2023	Dec. 31, 2022
Carrying Amount
Assets:
Cash and cash equivalents	$ 2,245.9	$ 1,223.5
Deposits with banks	55.9	63.4
Notes receivable, net	11.2	10.7
Secured financings receivable	694.4	422.7
Liabilities:
Secured financings payable	585.7	366.3
Notes and contracts payable	1,393.3	1,645.8
Estimated Fair Value
Assets:
Cash and cash equivalents	2,245.9	1,223.5
Deposits with banks	55.3	62.7
Notes receivable, net	10.8	10.6
Secured financings receivable	694.4	422.7
Liabilities:
Secured financings payable	585.7	366.3
Notes and contracts payable	1,185.4	1,404.4
Estimated Fair Value | Level 1
Assets:
Cash and cash equivalents	2,245.9	1,223.5
Deposits with banks	4.3	8.5
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	51.0	54.2
Notes receivable, net	0.0	0.0
Secured financings receivable	694.4	422.7
Liabilities:
Secured financings payable	585.7	366.3
Notes and contracts payable	1,182.7	1,400.6
Estimated Fair Value | Level 3
Assets:
Cash and cash equivalents	0.0	0.0
Deposits with banks	0.0	0.0
Notes receivable, net	10.8	10.6
Secured financings receivable	0.0	0.0
Liabilities:
Secured financings payable	0.0	0.0
Notes and contracts payable	$ 2.7	$ 3.8