Condensed Financial Information (Parent Company Only) - Condensed Balance Sheets (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Condensed balance sheets:
Cash and cash equivalents	$ 578.0	$ 870.5
Investment in subsidiaries, at equity:
Other assets	268.4	348.7
Assets	30,671.2	32,287.8
Long-term debt	99.0	98.9
Subordinated debentures held by subsidiary trusts	163.1	163.1
Total liabilities	27,443.7	29,214.0
Stockholders’ equity	3,227.5	3,073.8	$ 1,986.6	$ 1,959.8
Total liabilities and stockholders’ equity	30,671.2	32,287.8
Parent Company
Condensed balance sheets:
Cash and cash equivalents	275.7	154.8
Investment in subsidiaries, at equity:
Equity method investments	3,062.2	2,985.8
Advances from subsidiaries, net	43.5	0.1
Other assets	147.0	240.7
Assets	3,528.4	3,381.4
Other liabilities	38.8	45.6
Subordinated debentures held by subsidiary trusts	163.1	163.1
Total liabilities	300.9	307.6
Stockholders’ equity	3,227.5	3,073.8
Total liabilities and stockholders’ equity	$ 3,528.4	$ 3,381.4