Earnings per share (Details) $ / shares in Units, $ in Millions		3 Months Ended								12 Months Ended
	Apr. 27, 2016 shares	Dec. 31, 2018 $ / shares shares	Sep. 30, 2018 $ / shares	Jun. 30, 2018 $ / shares	Mar. 31, 2018 $ / shares	Dec. 31, 2017 $ / shares shares	Sep. 30, 2017 $ / shares	Jun. 30, 2017 $ / shares	Mar. 31, 2017 $ / shares	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2015 shares
Numerator:
Net (loss) income										$ (16.7)	$ (136.3)	$ 51.8
Less: Income attributable to non-controlling interests										(1.4)	(13.7)	(19.3)
Less: Accrued dividends on Series A redeemable preference shares										(2.6)	(6.1)
Less: Change in carrying value of Series B preference shares										(36.4)
Add: Redemption of Series A redeemable preference shares										(13.8)
Net income (loss) available to Sirius Group common shareholders										$ (43.3)	$ (156.1)	$ 32.5
Denominator:
Weighted average shares outstanding for basic and diluted earnings per share | shares										119,253,924	120,000,000	120,000,000
Earnings per share
Basic earnings per share | $ / shares		$ (1.31)	$ (0.23)	$ 0.78	$ 0.32	$ 0.13	$ (1.49)	$ (0.01)	$ 0.06	$ (0.36)	$ (1.30)	$ 0.27
Diluted earnings per share | $ / shares		$ (1.31)	$ (0.23)	$ 0.78	$ 0.32	$ 0.13	$ (1.49)	$ (0.01)	$ 0.06	$ (0.36)	$ (1.30)	$ 0.27
Stock split, share multiplied value	10,000
Common shares issued | shares	120,000,000	115,151,251				120,000,000				115,151,251	120,000,000	120,000,000	12,000
Common shares outstanding | shares	120,000,000	115,151,251				120,000,000				115,151,251	120,000,000
potentially dilutive securities | shares										23,433,114	0	0