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Variable Interest Entities (Details) $ in Millions				12 Months Ended
	May 07, 2020 USD ($)	Apr. 04, 2019 USD ($)	Jan. 24, 2018 USD ($)	Dec. 31, 2020 USD ($) director	May 06, 2020	Dec. 31, 2019 USD ($)	May 31, 2019 USD ($)	Apr. 03, 2019	Feb. 05, 2018
Variable Interest Entity [Line Items]
Percentage of par value of policy reinsured				15.00%
Other investments				$ 786.8		$ 676.3
Payments to acquire business		$ 81.4
Goodwill				$ 525.5		$ 394.7
HG Global
Variable Interest Entity [Line Items]
Percentage of par value of policy reinsured				15.00%
BAM
Variable Interest Entity [Line Items]
Number directors to designate for election | director				2
First Loss Reinsurance Treaty | HG Global
Variable Interest Entity [Line Items]
Percentage of par value of policy reinsured				15.00%
First Loss Reinsurance Treaty | BAM
Variable Interest Entity [Line Items]
Percentage of premiums				60.00%
Elementum Holdings, L.P.
Variable Interest Entity [Line Items]
Ownership interest (as a percent)				28.90%		30.00%	30.00%
Other investments				$ 55.1			$ 55.1
Kudu
Variable Interest Entity [Line Items]
Ownership interest (as a percent)		99.10%						49.50%	49.50%
Payments to acquire business		$ 81.4
Assets acquired		155.5
Goodwill		7.6
Intangible assets acquired		2.2
Liabilities acquired		0.8
Noncontrolling interest acquired		$ 1.5
Fully diluted ownership interest (as a percent)		85.40%						42.70%	42.70%
DavidShield & PassportCard
Variable Interest Entity [Line Items]
Ownership interest (as a percent)	53.80%			53.80%	50.00%	50.00%
Other investments				$ 104.4
Payments to acquire businesses, gross			$ 41.8
Payments to acquire business			$ 28.3
Additional investment in acquisition	$ 15.0
Elementum Holdings, L.P.
Variable Interest Entity [Line Items]
Other investments				$ 55.1		$ 55.1

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This element represents the fair value of the noncontrolling interest in the acquiree at the acquisition date.

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