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
	Jun. 30, 2020	Sep. 30, 2020	Dec. 31, 2019
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill, fair value		$ 540.1	$ 402.3
Goodwill, impairment loss			7.6
Goodwill, net carrying value		540.1	394.7
Other intangible assets, acquisition date fair value		285.7	292.1
Other intangible assets, accumulated amortization		43.3	29.3
Impairment of intangible assets, including goodwill		6.2	10.4
Other intangible assets, impairments		6.2	2.8
Other intangible assets, net carrying value		236.2	260.0
Goodwill and other intangible assets		776.3	654.7
Intangible assets and goodwill, fair value		825.8	694.4
Non-controlling interest
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill and other intangible assets		(27.1)	(23.4)
Total
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill and other intangible assets		749.2	631.3
Specialty Insurance Distribution (NSM)
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill, fair value		511.8	381.6
Goodwill, net carrying value		511.8	381.6
Other intangible assets, acquisition date fair value		266.5	271.7
Other intangible assets, accumulated amortization		41.8	27.9
Other intangible assets, impairments	$ 6.2	6.2	2.4
Other intangible assets, net carrying value		218.5	241.4
Goodwill, foreign currency translation gain (loss)		5.2	1.4
Other intangible assets, foreign currency translation gain (loss)		$ (0.5)	0.7
Specialty Insurance Distribution (NSM) | Customer relationships
Acquired Finite-Lived Intangible Assets [Line Items]
Other intangible assets, weighted average useful life (in years)		9 years
Other intangible assets, acquisition date fair value		$ 118.4	121.7
Other intangible assets, accumulated amortization		28.4	19.8
Other intangible assets, impairments		3.5	1.4
Other intangible assets, net carrying value		$ 86.5	100.5
Specialty Insurance Distribution (NSM) | Trade names
Acquired Finite-Lived Intangible Assets [Line Items]
Other intangible assets, weighted average useful life (in years)		19 years
Other intangible assets, acquisition date fair value		$ 60.8	61.9
Other intangible assets, accumulated amortization		7.1	5.3
Other intangible assets, impairments		1.0	0.4
Other intangible assets, net carrying value		52.7	56.2
Specialty Insurance Distribution (NSM) | Information technology
Acquired Finite-Lived Intangible Assets [Line Items]
Other intangible assets, acquisition date fair value		3.1	3.9
Other intangible assets, accumulated amortization		1.4	1.4
Other intangible assets, impairments		1.7	0.6
Other intangible assets, net carrying value		$ 0.0	1.9
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		3.9	0.7
Other intangible assets, impairments		0.0	0.0
Other intangible assets, net carrying value		$ 78.6	81.8
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
Other intangible assets, acquisition date fair value		$ 1.7	1.7
Other intangible assets, accumulated amortization		1.0	0.7
Other intangible assets, impairments		0.0	0.0
Other intangible assets, net carrying value		0.7	1.0
Kudu
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill, fair value		7.6	7.6
Goodwill, net carrying value		$ 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
Other intangible assets, accumulated amortization		0.5	0.2
Other intangible assets, impairments			0.0
Other intangible assets, net carrying value		1.7	2.0
Other Operations
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill, fair value		20.7	13.1
Goodwill, impairment loss			7.6
Goodwill, net carrying value		20.7	5.5
Other intangible assets, acquisition date fair value		17.0	18.2
Other intangible assets, accumulated amortization		1.0	1.2
Other intangible assets, impairments			0.4
Other intangible assets, net carrying value		$ 16.0	16.6
Other Operations | Customer relationships
Acquired Finite-Lived Intangible Assets [Line Items]
Other intangible assets, weighted average useful life (in years)		11 years
Other intangible assets, acquisition date fair value		$ 7.2	7.2
Other intangible assets, accumulated amortization		0.8	0.4
Other intangible assets, impairments			0.0
Other intangible assets, net carrying value		$ 6.4	6.8
Other Operations | Trade names
Acquired Finite-Lived Intangible Assets [Line Items]
Other intangible assets, weighted average useful life (in years)		10 years
Other intangible assets, acquisition date fair value		$ 1.0	1.7
Other intangible assets, accumulated amortization		0.1	0.5
Other intangible assets, impairments			0.2
Other intangible assets, net carrying value		$ 0.9	1.0
Other Operations | Information technology
Acquired Finite-Lived Intangible Assets [Line Items]
Other intangible assets, weighted average useful life (in years)		5 years
Other intangible assets, acquisition date fair value		$ 0.0	0.5
Other intangible assets, accumulated amortization		0.0	0.3
Other intangible assets, impairments			0.2
Other intangible assets, net carrying value		$ 0.0	0.0
Other Operations | Other Intangible Assets
Acquired Finite-Lived Intangible Assets [Line Items]
Other intangible assets, weighted average useful life (in years)		5 years
Other intangible assets, acquisition date fair value		$ 0.2	0.2
Other intangible assets, accumulated amortization		0.1	0.0
Other intangible assets, impairments			0.0
Other intangible assets, net carrying value		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
Other intangible assets, accumulated amortization		0.0	0.0
Other intangible assets, impairments			0.0
Other intangible assets, net carrying value		$ 8.6	$ 8.6