Commitments and Contingencies (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018
American International Group | NSM Insurance HoldCo, LLC
Loss Contingencies [Line Items]
Contingent liability	$ 82.5
Sirius Group | Swedish Tax Authority | Sirius Group
Loss Contingencies [Line Items]
Contingent liability	16.6		$ 17.3
Change in contingent liability	$ (0.7)
Scenario, Forecast | American International Group | NSM Insurance HoldCo, LLC
Loss Contingencies [Line Items]
Contingent liability		$ 82.5