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Goodwill and Other Intangible Assets - Schedule of Goodwill and Other Intangible Assets Rollforward (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended		12 Months Ended
	Sep. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill, beginning balance		$ 525.5		$ 525.5
Loss on assets held for sale				(30.2)		$ 0.0
Amortization of other intangible assets	$ (10.2)		$ (5.4)	(28.1)	$ (17.1)
Goodwill, ending balance	671.4			671.4		525.5
Fresh Insurance Services Group Limited
Acquired Finite-Lived Intangible Assets [Line Items]
Proceeds from sale of other assets		1.5
Goodwill
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill, beginning balance	629.4	525.5	518.2	525.5	394.7	394.7
Acquisitions of businesses	55.7		14.9	71.5	140.0
Attribution of acquisition date fair value estimates between goodwill and other intangible assets (3)	(9.3)		0.0	(9.3)	0.0
Foreign currency translation	(4.0)		7.0	(2.3)	5.2
Impairments				0.0	0.0
Loss on assets held for sale				(30.2)	0.0
Measurement period adjustments	(0.4)		0.0	(0.6)	0.2
Amortization of other intangible assets	0.0		0.0	0.0	0.0
Goodwill, ending balance	671.4		540.1	671.4	540.1	525.5
Goodwill | Other
Acquired Finite-Lived Intangible Assets [Line Items]
Acquisitions of businesses				15.8
Goodwill | Ark
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill acquired				116.8	0.0
Goodwill | J.C. Taylor
Acquired Finite-Lived Intangible Assets [Line Items]
Acquisitions of businesses				55.7
Other Intangible Assets
Acquired Finite-Lived Intangible Assets [Line Items]
Other intangible assets (excluding goodwill), beginning balance	414.8	256.9	241.7	256.9	260.0	260.0
Acquisitions of businesses	0.0		0.0	0.0	0.0
Attribution of acquisition date fair value estimates between goodwill and other intangible assets (3)	9.3		0.0	9.3	0.0
Foreign currency translation	(0.3)		(0.1)	(0.2)	(0.5)
Impairments				0.0	(6.2)
Loss on assets held for sale				0.0	0.0
Measurement period adjustments	0.0		0.0	0.0	0.0
Amortization of other intangible assets	(10.2)		(5.4)	(28.1)	(17.1)
Other intangible assets (excluding goodwill), ending balance	413.6		236.2	413.6	236.2	256.9
Other Intangible Assets | Ark
Acquired Finite-Lived Intangible Assets [Line Items]
Intangible assets acquired				175.7	0.0
Total Goodwill and Other Intangible Assets
Acquired Finite-Lived Intangible Assets [Line Items]
Other intangible assets, gross (including goodwill), beginning balance	1,044.2	$ 782.4	759.9	782.4	654.7	654.7
Acquisitions of businesses	55.7		14.9	71.5	140.0
Attribution of acquisition date fair value estimates between goodwill and other intangible assets (3)	0.0		0.0	0.0	0.0
Foreign currency translation	(4.3)		6.9	(2.5)	4.7
Impairments				0.0	(6.2)
Loss on assets held for sale				(30.2)	0.0
Measurement period adjustments	(0.4)		0.0	(0.6)	0.2
Amortization of other intangible assets	(10.2)		(5.4)	(28.1)	(17.1)
Other intangible assets, gross (including goodwill), ending balance	$ 1,085.0		$ 776.3	1,085.0	776.3	$ 782.4
Total Goodwill and Other Intangible Assets | Ark
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill and other intangible assets acquired				$ 292.5	$ 0.0

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