Condensed Financial Information (Parent Company Only) - Condensed Statements of Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Condensed Financial Statements, Captions
Salaries and benefits	$ 173.7	$ 155.3	$ 146.4
Interest expense	26.6	59.0	40.9
Acquisition related expenses	0.0	20.3	12.4
Earnings before income tax benefit	209.3	235.1	206.3
Income tax benefit	48.1	54.1	46.1
Net income	161.2	181.0	160.2
Parent Company
Condensed Financial Statements, Captions
Dividends from subsidiaries	130.0	178.0	148.5
Other interest income	0.1	0.3	0.1
Other income, primarily management fees from subsidiaries	28.7	25.9	17.0
Total income	158.8	204.2	165.6
Salaries and benefits	31.5	34.2	25.3
Interest expense	6.6	4.7	4.5
Acquisition related expenses	0.0	17.0	8.1
Other operating expenses, net	15.6	14.8	14.5
Total expenses	53.7	70.7	52.4
Earnings before income tax benefit	105.1	133.5	113.2
Income tax benefit	(6.1)	(11.9)	(9.3)
Income before undistributed earnings of subsidiaries	111.2	145.4	122.5
Undistributed earnings of subsidiaries	50.0	35.6	37.7
Net income	$ 161.2	$ 181.0	$ 160.2