Quarterly financial results (Unaudited) (Details) $ / shares in Units, $ in Millions	3 Months Ended				6 Months Ended	9 Months Ended	12 Months Ended
	Dec. 31, 2021 USD ($) segment $ / shares shares	Sep. 30, 2021 USD ($) $ / shares shares	Jun. 30, 2021 USD ($) $ / shares shares	Mar. 31, 2021 USD ($) $ / shares shares	Jun. 30, 2021 USD ($) $ / shares shares	Sep. 30, 2021 USD ($) $ / shares shares	Dec. 31, 2021 USD ($) segment $ / shares shares	Dec. 31, 2020 USD ($) $ / shares shares	Dec. 31, 2019 USD ($) $ / shares shares
Revenues
Net premiums earned	$ 519.9	$ 499.6	$ 452.3	$ 245.2	$ 697.5	$ 1,197.1	$ 1,717.0	$ 610.8	$ 700.1
Net realized and unrealized investment gains (losses)	(60.6)	(11.7)	23.9	31.5	55.4	43.7	(16.9)	69.2	15.3
Net realized and unrealized investment gains (losses) from related party investment funds	(97.2)	202.4	45.6	153.2	198.8	401.2	304.0	195.0	249.6
Other net investment income	6.6	9.1	7.9	1.8	9.7	18.8	25.4	14.7	17.6
Total realized and unrealized investment gains and net investment income	(151.2)	199.8	77.4	186.5	263.9	463.7	312.5	278.9	282.5
Other revenues	29.3	33.2	31.8	56.9	88.7	121.9	151.2	0.0	0.0
Total revenues	398.0	732.6	561.5	488.6	1,050.1	1,782.7	2,180.7	889.7	982.6
Expenses
Loss and loss adjustment expenses incurred, net	351.4	577.3	250.9	146.9	397.8	975.1	1,326.5	465.3	403.5
Acquisition costs, net	106.3	106.9	105.6	69.0	174.6	281.5	387.8	187.1	295.6
Other underwriting expenses	38.2	53.3	46.5	20.8	67.3	120.6	158.8	30.1	34.2
Net corporate and other expenses	72.1	59.9	55.7	78.9	134.6	194.5	266.6	41.9	36.2
Intangible asset amortization	1.8	2.0	1.3	0.8	2.1	4.1	5.9	0.0	0.0
Interest expense	9.6	9.7	9.8	4.9	14.7	24.4	34.0	8.2	8.2
Foreign exchange (gains) losses	(27.5)	(16.1)	12.0	(12.4)	(0.4)	(16.5)	(44.0)	5.2	3.6
Total expenses	551.9	793.0	481.8	308.9	790.7	1,583.7	2,135.6	737.8	781.3
Income before income tax (expense) benefit	(153.9)	(60.4)	79.7	179.7	259.4	199.0	45.1	151.9	201.3
Income tax (expense) benefit	17.1	13.0	(9.6)	(9.8)	(19.4)	(6.4)	10.7	(8.1)	(0.7)
Net income (loss)	(136.8)	(47.4)	70.1	169.9	240.0	192.6	55.8	143.8	200.6
Net (income) loss attributable to noncontrolling interests	0.5	3.4	(1.6)	0.0	(1.6)	1.8	2.3	(0.3)	0.0
Net income available to SiriusPoint	(136.3)	(44.0)	68.5	169.9	238.4	194.4	58.1	143.5	200.6
Dividends on Series B preference shares	(4.0)	(4.0)	(4.0)	(1.5)	(5.5)	(9.5)	(13.5)	0.0	0.0
Net income (loss) 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 (loss) available to SiriusPoint common shareholders	$ (140.3)	$ (48.0)	$ 64.5	$ 168.4	$ 232.9	$ 184.9	$ 44.6	$ 143.5	$ 200.6
Earnings (loss) per share available to SiriusPoint common shareholders
Basic earnings (loss) per share available to SiriusPoint common shareholders (in dollars per share) | $ / shares	$ (0.88)	$ (0.30)	$ 0.37	$ 1.37	$ 1.57	$ 1.18	$ 0.28	$ 1.54	$ 2.18
Diluted earnings (loss) per share available to SiriusPoint common shareholders (in dollars per share) | $ / shares	$ (0.88)	$ (0.34)	$ 0.37	$ 1.35	$ 1.55	$ 1.17	$ 0.27	$ 1.53	$ 2.16
Weighted average number of common shares used in the determination of earnings per share
Basic (in shares) | 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
Diluted (in shares) | 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
Number of operating segments | segment	2						2