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Commitments and Contingencies (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jun. 28, 2019
Loss Contingencies [Line Items]
Rent expense	$ 7.2	$ 5.5	$ 3.5
Capital Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
Capital lease payments due next year	7.1
Capital lease payments due in two years	6.1
Capital lease payments due in three years	5.7
Capital Lease payments due in four years and thereafter	18.4
Commitments to fund other-long term investments	183.4
Sirius Group | Sirius Group | Swedish Tax Authority
Capital Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
Loss contingency accrual	16.5	17.3
Loss contingency accrual, period increase (decrease)	(0.8)	$ 17.3
American International Group | NSM Insurance HoldCo, LLC
Capital Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
Purchase obligation	$ 82.5
Intangible assets acquired				$ 82.5

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