Condensed Financial Information (Parent Company Only) - Condensed Statements of Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Condensed Financial Statements, Captions
Salaries and benefits	$ 146.4	$ 122.7	$ 108.7
Interest expense	40.9	28.0	17.6
Acquisition related expenses	12.4	27.2	2.8
Earnings before income tax benefit	206.3	156.7	145.3
Income tax benefit	46.1	50.2	49.6
Net income	160.2	106.5	95.7
Parent Company
Condensed Financial Statements, Captions
Dividends from subsidiaries	148.5	150.0	140.0
Other interest income	0.1	0.1	0.0
Other income, primarily management fees from subsidiaries	17.0	18.0	15.1
Total income	165.6	168.1	155.1
Salaries and benefits	25.3	21.8	18.8
Interest expense	4.5	4.7	4.1
Acquisition related expenses	8.1	25.3	1.5
Other operating expenses, net	14.5	13.0	10.4
Total expenses	52.4	64.8	34.8
Earnings before income tax benefit	113.2	103.3	120.3
Income tax benefit	(9.3)	(14.2)	(7.7)
Income before undistributed earnings of subsidiaries	122.5	117.5	128.0
Undistributed earnings of subsidiaries	37.7	(11.0)	(32.4)
Net income	$ 160.2	$ 106.5	$ 95.6