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		9 Months Ended		12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2019
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill, acquisition date fair value	$ 701.6		$ 701.6		$ 525.5
Goodwill, impairments			30.2		0.0
Goodwill, net carrying value	671.4		671.4		525.5
Other intangible assets, acquisition date fair value	501.9		501.9		317.8
Intangible assets and goodwill, acquisition date fair value	1,203.5		1,203.5		843.3
Other intangible assets, accumulated amortization	82.1		82.1		54.7
Other intangible assets, impairments			6.2		6.2
Impairment of intangible assets, including goodwill			36.4		6.2
Other intangible assets, net carrying value	413.6		413.6		256.9
Goodwill
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill, impairments			30.2	$ 0.0
Goodwill, net carrying value	671.4	$ 540.1	671.4	540.1	525.5	$ 629.4	$ 518.2	$ 394.7
Goodwill, foreign currency translation gain (loss)	(4.0)	7.0	(2.3)	5.2
Other Intangible Assets
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill, impairments			0.0	0.0
Other intangible assets, foreign currency translation gain (loss)	(0.3)	$ (0.1)	(0.2)	$ (0.5)
Total Equity Including Noncontrolling Interest
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill and other intangible assets	1,085.0		1,085.0		782.4
Non-controlling interest
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill and other intangible assets	118.0		118.0		28.1
Total
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill and other intangible assets	967.0		967.0		754.3
Ark
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill, acquisition date fair value	116.8		116.8		0.0
Goodwill, impairments			0.0		0.0
Goodwill, net carrying value	116.8		116.8		0.0
Ark | Trade names
Acquired Finite-Lived Intangible Assets [Line Items]
Other intangible assets, acquisition date fair value	175.7		175.7		0.0
Other intangible assets, accumulated amortization	0.0		0.0		0.0
Other intangible assets, impairments			0.0		0.0
Other intangible assets, net carrying value	175.7		175.7		0.0
Specialty Insurance Distribution (NSM)
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill, acquisition date fair value	559.8		559.8		506.4
Goodwill, impairments			30.2		0.0
Goodwill, net carrying value	529.6		529.6		506.4
Other intangible assets, acquisition date fair value	288.0		288.0		288.9
Other intangible assets, accumulated amortization	76.6		76.6		52.3
Other intangible assets, impairments			6.2		6.2
Other intangible assets, net carrying value	205.2		205.2		230.4
Specialty Insurance Distribution (NSM) | Goodwill
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill, foreign currency translation gain (loss)			(2.3)		13.4
Specialty Insurance Distribution (NSM) | Other Intangible Assets
Acquired Finite-Lived Intangible Assets [Line Items]
Other intangible assets, foreign currency translation gain (loss)			$ (0.2)		1.6
Specialty Insurance Distribution (NSM) | Customer relationships
Acquired Finite-Lived Intangible Assets [Line Items]
Other intangible assets, weighted average useful life (in years)			8 years 10 months 24 days
Other intangible assets, acquisition date fair value	136.0		$ 136.0		136.2
Other intangible assets, accumulated amortization	50.6		50.6		36.7
Other intangible assets, impairments			3.5		3.5
Other intangible assets, net carrying value	81.9		$ 81.9		96.0
Specialty Insurance Distribution (NSM) | Trade names
Acquired Finite-Lived Intangible Assets [Line Items]
Other intangible assets, weighted average useful life (in years)			16 years
Other intangible assets, acquisition date fair value	65.3		$ 65.3		65.4
Other intangible assets, accumulated amortization	10.9		10.9		8.3
Other intangible assets, impairments			1.0		1.0
Other intangible assets, net carrying value	53.4		53.4		56.1
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		3.1
Other intangible assets, accumulated amortization	1.4		1.4		1.4
Other intangible assets, impairments			1.7		1.7
Other intangible assets, net carrying value	0.0		$ 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		82.5
Other intangible assets, accumulated amortization	13.1		13.1		4.9
Other intangible assets, impairments			0.0		0.0
Other intangible assets, net carrying value	69.4		$ 69.4		77.6
Specialty Insurance Distribution (NSM) | Other Intangible Assets
Acquired Finite-Lived Intangible Assets [Line Items]
Other intangible assets, weighted average useful life (in years)			3 years 4 months 24 days
Other intangible assets, acquisition date fair value	1.1		$ 1.1		1.7
Other intangible assets, accumulated amortization	0.6		0.6		1.0
Other intangible assets, impairments			0.0		0.0
Other intangible assets, net carrying value	0.5		0.5		0.7
Kudu
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill, acquisition date fair value	7.6		7.6		7.6
Goodwill, impairments			0.0		0.0
Goodwill, net carrying value	7.6		$ 7.6		7.6
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		2.2
Other intangible assets, accumulated amortization	0.8		0.8		0.6
Other intangible assets, impairments			0.0		0.0
Other intangible assets, net carrying value	1.4		1.4		1.6
Other Operations
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill, acquisition date fair value	17.4		17.4		11.5
Goodwill, impairments			0.0		0.0
Goodwill, net carrying value	17.4		17.4		11.5
Other intangible assets, acquisition date fair value	36.0		36.0		26.7
Other intangible assets, accumulated amortization	4.7		4.7		1.8
Other intangible assets, impairments			0.0		0.0
Other intangible assets, net carrying value	31.3		$ 31.3		24.9
Other Operations | Customer relationships
Acquired Finite-Lived Intangible Assets [Line Items]
Other intangible assets, weighted average useful life (in years)			13 years 3 months 18 days
Other intangible assets, acquisition date fair value	18.9		$ 18.9		14.2
Other intangible assets, accumulated amortization	3.5		3.5		1.4
Other intangible assets, impairments			0.0		0.0
Other intangible assets, net carrying value	15.4		$ 15.4		12.8
Other Operations | Trade names
Acquired Finite-Lived Intangible Assets [Line Items]
Other intangible assets, weighted average useful life (in years)			18 years 1 month 6 days
Other intangible assets, acquisition date fair value	8.2		$ 8.2		3.6
Other intangible assets, accumulated amortization	1.0		1.0		0.3
Other intangible assets, impairments			0.0		0.0
Other intangible assets, net carrying value	7.2		$ 7.2		3.3
Other Operations | Other Intangible Assets
Acquired Finite-Lived Intangible Assets [Line Items]
Other intangible assets, weighted average useful life (in years)			5 years 4 months 24 days
Other intangible assets, acquisition date fair value	0.3		$ 0.3		0.3
Other intangible assets, accumulated amortization	0.2		0.2		0.1
Other intangible assets, impairments			0.0		0.0
Other intangible assets, net carrying value	0.1		0.1		0.2
Other Operations | Insurance Licenses
Acquired Finite-Lived Intangible Assets [Line Items]
Other intangible assets, acquisition date fair value	8.6		8.6		8.6
Other intangible assets, accumulated amortization	0.0		0.0		0.0
Other intangible assets, impairments			0.0		0.0
Other intangible assets, net carrying value	$ 8.6		$ 8.6		$ 8.6