Earnings per share - Basic earnings per share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Numerator:
Net income	$ 8.2	$ 98.2	$ 112.3	$ 141.5
Less: Income attributable to non-controlling interests	(0.8)	(0.4)	(1.2)	(0.6)
Less: Change in carrying value of Series B preference shares	(0.8)		(9.2)
Less: Accrued dividends on Series A redeemable preference shares				(2.6)
Net income available for dividends out of undistributed earnings	6.6	97.8	101.9	138.3
Net income available to Sirius Group common shareholders	$ 6.0	$ 93.8	$ 92.4	$ 132.6
Denominator:
Weighted average shares outstanding for basic earnings per share	115,243,685	120,000,000	115,212,772	120,000,000
Earnings per share
Basic earnings per share	$ 0.05	$ 0.78	$ 0.80	$ 1.11
Series B preference shares
Numerator:
Less: Earnings attributable to preference shares	$ (0.6)		$ (9.5)
Series A redeemable preference shares
Numerator:
Less: Earnings attributable to preference shares		$ (4.0)		$ (5.7)