Segment Information - Summarized Financial Information Reportable Segments (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021	Dec. 31, 2021
Segment Reporting Information [Line Items]
Total revenues	$ 904.7	$ 815.3
Investment income	0.2	0.3
Amortization	31.1	29.5
Depreciation	8.1	7.5
Interest expense	18.3	16.3
Income before income taxes	265.0	239.2
Total assets	11,272.9	8,910.8	$ 9,795.4
Capital expenditures	10.0	11.4
Operating Segments | Retail
Segment Reporting Information [Line Items]
Total revenues	596.4	522.9
Investment income	0.0	0.0
Amortization	21.1	18.8
Depreciation	2.5	2.8
Interest expense	23.6	22.6
Income before income taxes	184.1	151.2
Total assets	4,903.9	7,210.5
Capital expenditures	1.6	2.3
Operating Segments | National Programs
Segment Reporting Information [Line Items]
Total revenues	162.2	154.9
Investment income	0.1	0.2
Amortization	6.7	6.9
Depreciation	2.8	2.1
Interest expense	2.2	4.1
Income before income taxes	41.7	41.1
Total assets	3,338.8	3,398.7
Capital expenditures	5.6	3.0
Operating Segments | Wholesale Brokerage
Segment Reporting Information [Line Items]
Total revenues	102.9	90.9
Investment income	0.1	0.1
Amortization	2.0	2.4
Depreciation	0.7	0.6
Interest expense	3.5	4.3
Income before income taxes	25.6	18.8
Total assets	1,124.8	1,787.0
Capital expenditures	0.4	0.5
Operating Segments | Services
Segment Reporting Information [Line Items]
Total revenues	43.6	47.0
Investment income	0.0	0.0
Amortization	1.3	1.4
Depreciation	0.4	0.4
Interest expense	0.6	0.8
Income before income taxes	6.6	9.3
Total assets	290.5	483.9
Capital expenditures	0.2	0.1
Segment Reconciling Items
Segment Reporting Information [Line Items]
Total revenues	(0.4)	(0.4)
Amortization	0.0
Depreciation	1.7	1.6
Interest expense	(11.6)	(15.5)
Income before income taxes	7.0	18.8
Total assets	1,614.9	(3,969.3)
Capital expenditures	$ 2.2	$ 5.5