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Commitments and Contingencies (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Loss Contingencies [Line Items]
Operating lease expense	$ 7.7	$ 7.2	$ 5.5
Finance Lease, Liability, Payment, Due [Abstract]
Due year one	7.7
Due year two	7.3
Due year three	7.1
Due after year three	23.1
Commitments to fund other-long term investments	297.8
Net loss (gain) from sale of discontinued operations, net of tax	2.3	(0.8)	17.2
Sirius Group
Finance Lease, Liability, Payment, Due [Abstract]
Net loss (gain) from sale of discontinued operations, net of tax			17.3
Sirius Group | Swedish Tax Authority
Finance Lease, Liability, Payment, Due [Abstract]
Loss contingency due to foreign currency translations	(2.3)	0.8	$ (17.3)
Sirius Group | Sirius Group | Swedish Tax Authority
Finance Lease, Liability, Payment, Due [Abstract]
Loss contingency accrual	18.7	$ 16.5
Loss contingency due to foreign currency translations	$ 2.2

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Amount after tax of gain (loss) not previously recognized resulting from the disposal of a discontinued operation.

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