Goodwill and Other Intangible Assets - Schedule of Acquisition Date Fair Values, Accumulated Amortization and Net Carrying Values for Other Intangibles Assets and Goodwill, By Company (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2022	Mar. 31, 2021	Dec. 31, 2021	Dec. 31, 2020
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill, acquisition date fair value	$ 671.6		$ 675.7
Goodwill, impairments	30.2		30.2
Goodwill, net carrying value	641.4		645.5
Goodwill
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill, impairments	0.0	$ 30.2
Goodwill, net carrying value	641.4	613.4	645.5	$ 525.5
Goodwill, foreign currency translation gain (loss)	(4.1)	1.5
Other
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill, impairments	0.0	0.0
Other intangible assets, foreign currency translation gain (loss)	(0.2)	$ 0.2
Total Equity Including Noncontrolling Interest
Acquired Finite-Lived Intangible Assets [Line Items]
Other intangible assets, acquisition date fair value	1,191.5		1,196.1
Other intangible assets, accumulated amortization	103.6		93.8
Impairment of intangible assets, including goodwill	36.4		36.4
Goodwill and other intangible assets	1,051.5		1,065.9
Non-controlling interest
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill and other intangible assets	117.1		117.6
Total
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill and other intangible assets	934.4		948.3
Other
Acquired Finite-Lived Intangible Assets [Line Items]
Other intangible assets, acquisition date fair value	519.9		520.4
Other intangible assets, accumulated amortization	103.6		93.8
Other intangible assets, impairments	6.2		6.2
Other intangible assets, net carrying value	410.1		420.4
Ark
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill, acquisition date fair value	116.8		116.8
Goodwill, impairments	0.0		0.0
Goodwill, net carrying value	116.8		116.8
Ark | Trade names
Acquired Finite-Lived Intangible Assets [Line Items]
Other intangible assets, acquisition date fair value	175.7		175.7
Other intangible assets, accumulated amortization	0.0		0.0
Other intangible assets, impairments	0.0		0.0
Other intangible assets, net carrying value	175.7		175.7
Specialty Insurance Distribution (NSM)
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill, acquisition date fair value	529.3		533.4
Goodwill, impairments	30.2		30.2
Goodwill, net carrying value	499.1		503.2
Other intangible assets, acquisition date fair value	314.7		315.2
Other intangible assets, accumulated amortization	95.6		86.8
Other intangible assets, impairments	6.2		6.2
Other intangible assets, net carrying value	212.9		222.2
Specialty Insurance Distribution (NSM) | Goodwill
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill, foreign currency translation gain (loss)	(4.1)
Other intangible assets, foreign currency translation gain (loss)	(0.5)
Specialty Insurance Distribution (NSM) | Other
Acquired Finite-Lived Intangible Assets [Line Items]
Other intangible assets, foreign currency translation gain (loss)	$ 0.3
Specialty Insurance Distribution (NSM) | Customer relationships
Acquired Finite-Lived Intangible Assets [Line Items]
Other intangible assets, weighted average useful life (in years)	9 years 2 months 12 days
Other intangible assets, acquisition date fair value	$ 159.4		159.9
Other intangible assets, accumulated amortization	62.3		57.0
Other intangible assets, impairments	3.5		3.5
Other intangible assets, net carrying value	$ 93.6		99.4
Specialty Insurance Distribution (NSM) | Trade names
Acquired Finite-Lived Intangible Assets [Line Items]
Other intangible assets, weighted average useful life (in years)	18 years
Other intangible assets, acquisition date fair value	$ 68.5		68.6
Other intangible assets, accumulated amortization	12.9		11.9
Other intangible assets, impairments	1.0		1.0
Other intangible assets, net carrying value	54.6		55.7
Specialty Insurance Distribution (NSM) | Information technology platform
Acquired Finite-Lived Intangible Assets [Line Items]
Other intangible assets, acquisition date fair value	3.1		3.1
Other intangible assets, accumulated amortization	1.4		1.4
Other intangible assets, impairments	1.7		1.7
Other intangible assets, net carrying value	$ 0.0		0.0
Specialty Insurance Distribution (NSM) | Renewal rights
Acquired Finite-Lived Intangible Assets [Line Items]
Other intangible assets, weighted average useful life (in years)	12 years
Other intangible assets, acquisition date fair value	$ 82.5		82.5
Other intangible assets, accumulated amortization	18.3		15.8
Other intangible assets, impairments	0.0		0.0
Other intangible assets, net carrying value	$ 64.2		66.7
Specialty Insurance Distribution (NSM) | Other
Acquired Finite-Lived Intangible Assets [Line Items]
Other intangible assets, weighted average useful life (in years)	3 years 3 months 18 days
Other intangible assets, acquisition date fair value	$ 1.2		1.1
Other intangible assets, accumulated amortization	0.7		0.7
Other intangible assets, impairments	0.0		0.0
Other intangible assets, net carrying value	0.5		0.4
Asset Management (Kudu)
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill, acquisition date fair value	7.6		7.6
Goodwill, impairments	0.0		0.0
Goodwill, net carrying value	$ 7.6		7.6
Asset Management (Kudu) | Trade names
Acquired Finite-Lived Intangible Assets [Line Items]
Other intangible assets, weighted average useful life (in years)	7 years
Other intangible assets, acquisition date fair value	$ 2.2		2.2
Other intangible assets, accumulated amortization	0.9		0.9
Other intangible assets, impairments	0.0		0.0
Other intangible assets, net carrying value	1.3		1.3
Other Operations
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill, acquisition date fair value	17.9		17.9
Goodwill, impairments	0.0		0.0
Goodwill, net carrying value	17.9		17.9
Other intangible assets, acquisition date fair value	27.3		27.3
Other intangible assets, accumulated amortization	7.1		6.1
Other intangible assets, impairments	0.0		0.0
Other intangible assets, net carrying value	$ 20.2		21.2
Other Operations | Customer relationships
Acquired Finite-Lived Intangible Assets [Line Items]
Other intangible assets, weighted average useful life (in years)	13 years
Other intangible assets, acquisition date fair value	$ 18.8		18.8
Other intangible assets, accumulated amortization	5.2		4.5
Other intangible assets, impairments	0.0		0.0
Other intangible assets, net carrying value	$ 13.6		14.3
Other Operations | Trade names
Acquired Finite-Lived Intangible Assets [Line Items]
Other intangible assets, weighted average useful life (in years)	17 years 6 months
Other intangible assets, acquisition date fair value	$ 8.2		8.2
Other intangible assets, accumulated amortization	1.7		1.5
Other intangible assets, impairments	0.0		0.0
Other intangible assets, net carrying value	$ 6.5		6.7
Other Operations | Other
Acquired Finite-Lived Intangible Assets [Line Items]
Other intangible assets, weighted average useful life (in years)	5 years 3 months 18 days
Other intangible assets, acquisition date fair value	$ 0.3		0.3
Other intangible assets, accumulated amortization	0.2		0.1
Other intangible assets, impairments	0.0		0.0
Other intangible assets, net carrying value	$ 0.1		$ 0.2