Goodwill and Other Intangible Assets - Schedule of Goodwill and Other Intangible Assets Rollforward (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Acquired Finite-Lived Intangible Assets [Line Items]
Beginning balance	$ 525.5	$ 525.5
Amortization		(39.8)	$ (28.3)	$ (21.8)
Impairments		(30.2)	0.0
Ending balance		645.5	525.5
J.C. Taylor Insurance
Acquired Finite-Lived Intangible Assets [Line Items]
Attribution of acquisition date fair value estimates between goodwill and other intangible assets		27.2
Kingsbridge Group Limited
Acquired Finite-Lived Intangible Assets [Line Items]
Attribution of acquisition date fair value estimates between goodwill and other intangible assets			13.6
Measurement period adjustments			4.1
Other Operations
Acquired Finite-Lived Intangible Assets [Line Items]
Attribution of acquisition date fair value estimates between goodwill and other intangible assets		9.3	9.6
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]
Beginning balance	525.5	525.5	394.7
Acquisition of businesses		71.5	140.0
Attribution of acquisition date fair value estimates between goodwill and other intangible assets		(36.5)	(23.2)
Measurement period adjustments		(0.1)	0.6
Disposition of businesses		0.0	0.0
Amortization		0.0	0.0
Impairments		0.0	0.0
Assets held for sale			0.0
Loss on assets held for sale		(30.2)	0.0
Foreign currency translation		(1.5)	13.4
Ending balance		645.5	525.5	394.7
Goodwill | Ark
Acquired Finite-Lived Intangible Assets [Line Items]
Ark Transaction		116.8	0.0
Goodwill | J.C. Taylor Insurance
Acquired Finite-Lived Intangible Assets [Line Items]
Acquisition of businesses		55.7
Goodwill | Kingsbridge Group Limited
Acquired Finite-Lived Intangible Assets [Line Items]
Acquisition of businesses			125.1
Goodwill | Other Operations
Acquired Finite-Lived Intangible Assets [Line Items]
Acquisition of businesses			14.9
Goodwill | Other Operations
Acquired Finite-Lived Intangible Assets [Line Items]
Acquisition of businesses		15.8
Total other intangible assets
Acquired Finite-Lived Intangible Assets [Line Items]
Beginning balance	256.9	256.9	260.0
Acquisition of businesses		0.0	0.0
Attribution of acquisition date fair value estimates between goodwill and other intangible assets		36.5	23.2
Measurement period adjustments		0.0	6.6
Disposition of businesses		0.0	0.0
Amortization		(39.8)	(28.3)
Impairments		0.0	(6.2)
Assets held for sale		(8.6)	0.0
Loss on assets held for sale		0.0	0.0
Foreign currency translation		(0.3)	1.6
Ending balance		420.4	256.9	260.0
Total other intangible assets | Ark
Acquired Finite-Lived Intangible Assets [Line Items]
Ark Transaction		175.7	0.0
Total Goodwill and Other Intangible Assets
Acquired Finite-Lived Intangible Assets [Line Items]
Beginning balance	$ 782.4	782.4	654.7
Acquisition of businesses		71.5	140.0
Attribution of acquisition date fair value estimates between goodwill and other intangible assets		0.0	0.0
Measurement period adjustments		(0.1)	7.2
Disposition of businesses		0.0	0.0
Amortization		(39.8)	(28.3)
Impairments		0.0	(6.2)
Assets held for sale		(8.6)	0.0
Loss on assets held for sale		(30.2)	0.0
Foreign currency translation		(1.8)	15.0
Ending balance		1,065.9	782.4	$ 654.7
Total Goodwill and Other Intangible Assets | Ark
Acquired Finite-Lived Intangible Assets [Line Items]
Ark Transaction		$ 292.5	$ 0.0