Goodwill and Other Intangible Assets - Summary of Goodwill and Intangible Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Goodwill [Line Items]
Goodwill, fair value	$ 525.5	$ 402.3
Goodwill, impairment loss	0.0	7.6
Goodwill	525.5	394.7
Other intangible assets, fair value	843.3	694.4
Other intangible assets, accumulated amortization	54.7	29.3
Other intangible assets, net	248.3
Goodwill and intangible asset impairment	6.2	10.4
Goodwill and other intangible assets	782.4	656.7
NSM
Goodwill [Line Items]
Goodwill, fair value	506.4	381.6
Goodwill, impairment loss	0.0	0.0
Goodwill	506.4	381.6
Other intangible assets, fair value	288.9	271.7
Other intangible assets, accumulated amortization	52.3	27.9
Other intangible assets, net	230.4	241.4
NSM | Fresh Insurance Services Group Limited
Goodwill [Line Items]
Impairments	6.2	2.4
Kudu
Goodwill [Line Items]
Goodwill, fair value	7.6	7.6
Goodwill, impairment loss	0.0	0.0
Goodwill	7.6	7.6
Other
Goodwill [Line Items]
Goodwill, fair value	11.5	13.1
Goodwill, impairment loss	0.0	7.6
Goodwill	11.5	5.5
Other intangible assets, fair value	26.7	18.2
Other intangible assets, accumulated amortization	1.8	1.2
Impairments		0.4
Other intangible assets, net	$ 24.9	16.6
Customer relationships | NSM
Goodwill [Line Items]
Acquired other intangible assets, weighted average useful life (in years)	8 years 10 months 24 days
Other intangible assets, fair value	$ 136.2	121.7
Other intangible assets, accumulated amortization	36.7	19.8
Other intangible assets, net	96.0	100.5
Customer relationships | NSM | Fresh Insurance Services Group Limited
Goodwill [Line Items]
Impairments	$ 3.5	1.4
Customer relationships | Other
Goodwill [Line Items]
Acquired other intangible assets, weighted average useful life (in years)	10 years 8 months 12 days
Other intangible assets, fair value	$ 14.2	7.2
Other intangible assets, accumulated amortization	1.4	0.4
Impairments	0.0	0.0
Other intangible assets, net	$ 12.8	6.8
Trade names | NSM
Goodwill [Line Items]
Acquired other intangible assets, weighted average useful life (in years)	16 years
Other intangible assets, fair value	$ 65.4	61.9
Other intangible assets, accumulated amortization	8.3	5.3
Other intangible assets, net	56.1	56.2
Trade names | NSM | Fresh Insurance Services Group Limited
Goodwill [Line Items]
Impairments	$ 1.0	0.4
Trade names | Kudu
Goodwill [Line Items]
Acquired other intangible assets, weighted average useful life (in years)	7 years
Other intangible assets, fair value	$ 2.2	2.2
Other intangible assets, accumulated amortization	0.6	0.2
Impairments	0.0	0.0
Other intangible assets, net	$ 1.6	2.0
Trade names | Other
Goodwill [Line Items]
Acquired other intangible assets, weighted average useful life (in years)	9 years 7 months 6 days
Other intangible assets, fair value	$ 3.6	1.7
Other intangible assets, accumulated amortization	0.3	0.5
Impairments	0.0	0.2
Other intangible assets, net	$ 3.3	1.0
Information technology platform | NSM
Goodwill [Line Items]
Acquired other intangible assets, weighted average useful life (in years)	0 years
Other intangible assets, fair value	$ 3.1	3.9
Other intangible assets, accumulated amortization	1.4	1.4
Other intangible assets, net	0.0	1.9
Information technology platform | NSM | Fresh Insurance Services Group Limited
Goodwill [Line Items]
Impairments	$ 1.7	0.6
Information technology platform | Other
Goodwill [Line Items]
Acquired other intangible assets, weighted average useful life (in years)	5 years
Other intangible assets, fair value	$ 0.0	0.5
Other intangible assets, accumulated amortization	0.0	0.3
Impairments	0.0	0.2
Other intangible assets, net	0.0	0.0
Insurance licenses | Other
Goodwill [Line Items]
Other intangible assets, fair value	8.6	8.6
Other intangible assets, accumulated amortization	0.0	0.0
Impairments	0.0	0.0
Other intangible assets, net	$ 8.6	8.6
Renewal rights | NSM
Goodwill [Line Items]
Acquired other intangible assets, weighted average useful life (in years)	12 years
Other intangible assets, fair value	$ 82.5	82.5
Other intangible assets, accumulated amortization	4.9	0.7
Impairments	0.0	0.0
Other intangible assets, net	$ 77.6	81.8
Other | NSM
Goodwill [Line Items]
Acquired other intangible assets, weighted average useful life (in years)	3 years 4 months 24 days
Other intangible assets, fair value	$ 1.7	1.7
Other intangible assets, accumulated amortization	1.0	0.7
Impairments	0.0	0.0
Other intangible assets, net	$ 0.7	1.0
Other | Other
Goodwill [Line Items]
Acquired other intangible assets, weighted average useful life (in years)	5 years 4 months 24 days
Other intangible assets, fair value	$ 0.3	0.2
Other intangible assets, accumulated amortization	0.1	0.0
Impairments	0.0	0.0
Other intangible assets, net	0.2	0.2
Total other intangible assets
Goodwill [Line Items]
Other intangible assets, fair value	317.8	292.1
Other intangible assets, accumulated amortization	54.7	29.3
Impairments	6.2	2.8
Other intangible assets, net	256.9	260.0
Non-controlling Interests
Goodwill [Line Items]
Goodwill and other intangible assets	(28.1)	(23.4)
Total
Goodwill [Line Items]
Goodwill and other intangible assets	754.3	631.3
Goodwill
Goodwill [Line Items]
Goodwill	525.5	394.7	$ 379.9
Goodwill, foreign currency translation gain	13.4	1.8
Goodwill | NSM
Goodwill [Line Items]
Goodwill, foreign currency translation gain	13.4	1.8
Total other intangible assets | NSM
Goodwill [Line Items]
Goodwill, foreign currency translation gain	$ 1.6	$ 0.7