Earnings per share available to SiriusPoint common shareholders - Computation of basic and diluted earnings per share available to SiriusPoint common shareholders (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended				6 Months Ended	9 Months Ended	12 Months Ended
	Dec. 31, 2021	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Jun. 30, 2021	Sep. 30, 2021	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Weighted-average number of common shares outstanding:
Basic number of common shares outstanding (in shares)	159,268,777	159,225,772	158,832,629	116,760,760	137,912,915	145,095,270	148,667,770	92,510,090	91,835,990
Dilutive effect of warrants (in shares)							0	0	91,884
Diluted number of common shares outstanding (in shares)	159,268,777	160,240,888	160,894,216	118,146,341	139,561,196	147,597,964	150,156,466	92,957,799	92,652,316
Basic earnings per common share:
Net income available to SiriusPoint common shareholders	$ (140.3)	$ (48.0)	$ 64.5	$ 168.4	$ 232.9	$ 184.9	$ 44.6	$ 143.5	$ 200.6
Net income allocated to SiriusPoint participating common shareholders							(3.4)	(1.1)	(0.6)
Net income allocated to SiriusPoint common shareholders							$ 41.2	$ 142.4	$ 200.0
Basic earnings per share available to SiriusPoint common shareholders (in dollars per share)	$ (0.88)	$ (0.30)	$ 0.37	$ 1.37	$ 1.57	$ 1.18	$ 0.28	$ 1.54	$ 2.18
Diluted earnings per common share:
Net income available to SiriusPoint common shareholders	$ (140.3)	$ (48.0)	$ 64.5	$ 168.4	$ 232.9	$ 184.9	$ 44.6	$ 143.5	$ 200.6
Net income allocated to SiriusPoint participating common shareholders							(3.4)	(1.1)	(0.6)
Net income allocated to SiriusPoint common shareholders							$ 41.2	$ 142.4	$ 200.0
Diluted earnings per share available to SiriusPoint common shareholders (in dollars per share)	$ (0.88)	$ (0.34)	$ 0.37	$ 1.35	$ 1.55	$ 1.17	$ 0.27	$ 1.53	$ 2.16
Options
Weighted-average number of common shares outstanding:
Dilutive effect of share-based payment awards (in shares)							0	0	125,530
Restricted share units
Weighted-average number of common shares outstanding:
Dilutive effect of share-based payment awards (in shares)							1,488,696	447,709	598,912