Condensed Financial Information (Parent Company Only) - Condensed Statements of Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Condensed Financial Statements, Captions
Salaries and benefits	$ 164.9	$ 173.7	$ 155.3
Interest expense	17.3	26.6	59.0
Acquisition related expenses	11.6	0.0	20.3
Earnings before income tax benefit		209.3	235.1
Income tax benefit	55.7	48.1	54.1
Net income	192.1	161.2	181.0
Parent Company
Condensed Financial Statements, Captions
Dividends from subsidiaries	160.0	130.0	178.0
Other interest income	0.0	0.1	0.3
Other income, primarily management fees from subsidiaries	41.3	28.7	25.9
Total income	201.3	158.8	204.2
Salaries and benefits	36.4	31.5	34.2
Interest expense	8.2	6.6	4.7
Acquisition related expenses	11.6	0.0	17.0
Other operating expenses, net	17.6	15.6	14.8
Total expenses	73.8	53.7	70.7
Earnings before income tax benefit	127.5	105.1	133.5
Income tax benefit	(7.6)	(6.1)	(11.9)
Income before undistributed earnings of subsidiaries	135.1	111.2	145.4
Undistributed earnings of subsidiaries	57.0	50.0	35.6
Net income	$ 192.1	$ 161.2	$ 181.0