Earnings per share - Basic and Diluted earnings per share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Numerator:
Net (loss) income	$ (7.2)	$ 8.2	$ (128.5)	$ 112.3
Less: Income attributable to non-controlling interests	0.2	(0.8)	0.0	(1.2)
Less: Change in carrying value of Series B preference shares	(6.6)	(0.8)	16.8	(9.2)
Net (loss) income attributable to Sirius Group's common shareholders	(13.6)	6.6	(111.7)	101.9
Less: Earnings attributable to Series B preference shares	0.0	(0.6)	0.0	(9.5)
Net income available to Sirius Group common shareholders	(13.6)	6.0	(111.7)	92.4
Add: Change in carrying value of Series B preference shares	0.0	0.0	(16.8)	9.2
Net income available to Sirius Group common shareholders on a diluted basis	$ (13.6)	$ 6.0	$ (128.5)	$ 101.6
Denominator:
Weighted average shares outstanding for basic earnings per share (in shares)	115,278,176	115,243,685	115,269,720	115,212,772
Add: Series B preference shares (in shares)	0	0	11,901,670	11,901,670
Add: Unvested performance share units and restricted share units (in shares)	0	552,682	0	427,960
Weighted average shares outstanding for diluted earnings per share (in shares)	115,278,176	115,796,367	127,171,390	127,542,402
Earnings per share
Basic earnings per share (in dollars per share)	$ (0.12)	$ 0.05	$ (0.97)	$ 0.80
Diluted earnings per share (in dollars per share)	$ (0.12)	$ 0.05	$ (1.01)	$ 0.80
Potentially dilutive securities excluded from the calculation of diluted earnings per share (in shares)	27,825,191	15,923,521	30,933,781	19,156,833