Condensed Financial Information (Parent Company Only) - Condensed Statements of Income (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended	12 Months Ended
	Mar. 31, 2018	Sep. 30, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Condensed Financial Statements, Captions
Salaries and benefits			$ 155.3	$ 146.4	$ 122.7
Interest expense			59.0	40.9	28.0
Acquisition related expenses		$ 12.4	20.3	12.4	27.2
Earnings before income tax benefit			235.1	206.3	156.7
Income tax benefit			54.1	46.1	50.2
Net income	$ 160.2		181.0	160.2	106.5
Parent Company
Condensed Financial Statements, Captions
Dividends from subsidiaries			178.0	148.5	150.0
Other interest income			0.3	0.1	0.1
Other income, primarily management fees from subsidiaries			25.9	17.0	18.0
Total income			204.2	165.6	168.1
Salaries and benefits			34.2	25.3	21.8
Interest expense			4.7	4.5	4.7
Acquisition related expenses			17.0	8.1	25.3
Other operating expenses, net			14.8	14.5	13.0
Total expenses			70.7	52.4	64.8
Earnings before income tax benefit			133.5	113.2	103.3
Income tax benefit			(11.9)	(9.3)	(14.2)
Income before undistributed earnings of subsidiaries			145.4	122.5	117.5
Undistributed earnings of subsidiaries			35.6	37.7	(11.0)
Net income			$ 181.0	$ 160.2	$ 106.5