Condensed Financial Information (Parent Company Only) - Condensed Balance Sheets (Details) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Condensed balance sheets:
Cash and cash equivalents	$ 896.6	$ 578.0
Investment in subsidiaries, at equity:
Other assets	304.0	268.4
Assets	29,137.4	30,671.2
Subordinated debentures held by subsidiary trusts	163.1	163.1
Total liabilities	25,833.4	27,443.7
Stockholders’ equity	3,304.0	3,227.5	$ 3,073.8	$ 1,986.6
Total liabilities and stockholders’ equity	29,137.4	30,671.2
Parent Company
Condensed balance sheets:
Cash and cash equivalents	273.4	275.7
Investment in subsidiaries, at equity:
Equity method investments	3,160.5	3,062.2
Advances from subsidiaries, net	46.8	43.5
Other assets	137.9	147.0
Assets	3,618.6	3,528.4
Other liabilities	52.4	38.8
Long-term debt	99.1	99.0
Subordinated debentures held by subsidiary trusts	163.1	163.1
Total liabilities	314.6	300.9
Stockholders’ equity	3,304.0	3,227.5
Total liabilities and stockholders’ equity	$ 3,618.6	$ 3,528.4