Earnings per share - Basic and Diluted earnings per share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Numerator:
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	$ (104.0)	$ (7.6)	$ (232.5)	$ 104.7
Less: Income attributable to non-controlling interests	(0.2)	(0.4)	(0.2)	(1.6)
Less: Change in carrying value of Series B preference shares	8.7	5.3	25.5	(3.9)
Net (loss) income attributable to Sirius Group's common shareholders	(95.5)	(2.7)	(207.2)	99.2
Less: Earnings attributable to Series B preference shares	0.0	0.0	0.0	(9.3)
Net income available to Sirius Group common shareholders	(95.5)	(2.7)	(207.2)	89.9
Add: Change in carrying value of Series B preference shares	0.0	(5.3)	(25.5)	0.0
Net income available to Sirius Group common shareholders on a diluted basis	$ (95.5)	$ (8.0)	$ (232.7)	$ 89.9
Denominator:
Basic earnings per share (in dollars per share)	$ (0.83)	$ (0.02)	$ (1.80)	$ 0.78
Weighted average shares outstanding for basic earnings per share (in shares)	115,297,945	115,251,853	115,279,197	115,225,942
Add: Series B preference shares (in shares)	0	11,901,670	11,901,670	0
Add: Unvested performance share units and restricted share units (in shares)	0	0	0	393,280
Weighted average shares outstanding for diluted earnings per share (in shares)	115,297,945	127,153,523	127,180,867	115,619,222
Diluted earnings per share (in dollars per share)	$ (0.83)	$ (0.06)	$ (1.83)	$ 0.78
Potentially dilutive securities excluded from the calculation of diluted earnings per share (in shares)	26,057,275	19,537,270	14,155,605	31,045,660