Earnings per share - Basic earnings per share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Numerator:
Net income	$ 104.1	$ 43.3
Less: Income attributable to non-controlling interests	(0.4)	(0.2)
Less: Change in carrying value of Series B preference shares	(8.4)
Less: Accrued dividends on Series A redeemable preference shares		(2.6)
Net income available for dividends out of undistributed earnings	95.3	40.5
Net income available to Sirius Group common shareholders	$ 86.4	$ 38.8
Denominator:
Weighted average shares outstanding for basic earnings per share	115,182,331	120,000,000
Earnings per share
Basic earnings per share	$ 0.75	$ 0.32
Series B preference shares
Numerator:
Less: Earnings attributable to preference shares	$ (8.9)
Series A redeemable preference shares
Numerator:
Less: Earnings attributable to preference shares		$ (1.7)