Condensed Financial Information (Parent Company Only) - Condensed Statements of Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Condensed Financial Statements, Captions
Salaries and benefits	$ 282.1	$ 164.9	$ 173.7
Interest expense	78.9	17.3	26.6
Acquisition related expenses	118.9	11.6	0.0
Earnings before income tax benefit	257.1	247.8	209.3
Income tax benefit	54.9	55.7	48.1
Net income	202.2	192.1	161.2
Parent Company
Condensed Financial Statements, Captions
Dividends from subsidiaries	360.0	160.0	130.0
Other interest income	0.4	0.0	0.1
Other income, primarily management fees from subsidiaries	51.7	41.3	28.7
Total income	412.1	201.3	158.8
Salaries and benefits	40.6	36.4	31.5
Interest expense	12.6	8.2	6.6
Acquisition related expenses	62.3	11.6	0.0
Other operating expenses, net	25.0	17.6	15.6
Total expenses	140.5	73.8	53.7
Earnings before income tax benefit	271.6	127.5	105.1
Income tax benefit	(18.9)	(7.6)	(6.1)
Income before undistributed earnings of subsidiaries	290.5	135.1	111.2
Undistributed (loss) earnings of subsidiaries	(88.3)	57.0	50.0
Net income	$ 202.2	$ 192.1	$ 161.2