Condensed Financial Information (Parent Company Only) - Condensed Statements of Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Condensed Financial Statements, Captions
Salaries and benefits	$ 263.1	$ 282.1	$ 164.9
Interest expense	401.3	78.9	17.3
Acquisition related expenses	0.0	118.9	11.6
Income before income tax	336.8	257.1	247.8
Income tax benefit	79.3	54.9	55.7
Net income	257.5	202.2	192.1
Parent Company
Condensed Financial Statements, Captions
Dividends from subsidiaries	360.0	360.0	160.0
Other interest income	0.5	0.4	0.0
Other income, primarily management fees from subsidiaries	51.6	51.7	41.3
Total income	412.1	412.1	201.3
Salaries and benefits	34.5	40.6	36.4
Interest expense	18.4	12.6	8.2
Acquisition related expenses	0.0	62.3	11.6
Other operating expenses, net	22.5	25.0	17.6
Total expenses	75.4	140.5	73.8
Income before income tax	336.7	271.6	127.5
Income tax benefit	(4.2)	(18.9)	(7.6)
Income before undistributed earnings of subsidiaries	340.9	290.5	135.1
Undistributed (loss) earnings of subsidiaries	(83.4)	(88.3)	57.0
Net income	$ 257.5	$ 202.2	$ 192.1