Segment reporting - Operating segment results (Details) - USD ($) $ 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
Segment Reporting Information [Line Items]
Gross premiums written							$ 2,236.5	$ 588.5	$ 668.4
Net premiums written							1,734.2	542.2	657.0
Net premiums earned	$ 519.9	$ 499.6	$ 452.3	$ 245.2	$ 697.5	$ 1,197.1	1,717.0	610.8	700.1
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 underwriting income (loss)							(156.1)	(71.7)	(33.2)
Segment income (loss)							(156.1)	(71.7)
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 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
Other revenues	29.3	33.2	31.8	56.9	88.7	121.9	151.2	0.0	0.0
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)
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 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
Underwriting Ratios:
Loss ratio							77.30%	76.20%	57.60%
Acquisition cost ratio							22.60%	30.60%	42.20%
Other underwriting expenses ratio							9.20%	4.90%	4.90%
Combined ratio							109.10%	111.70%	104.70%
Other long-term investments	$ 456.1						$ 456.1	$ 4.0
Operating Segments
Segment Reporting Information [Line Items]
Gross premiums written							2,248.3	559.6	$ 580.8
Net premiums written							1,777.7	513.3	569.4
Net premiums earned							1,733.7	582.7	611.5
Loss and loss adjustment expenses incurred, net							1,320.2	465.4	408.2
Acquisition costs, net							452.4	161.8	204.6
Other underwriting expenses							134.7	24.2	25.1
Net underwriting income (loss)							(173.6)	(68.7)	(26.4)
Segment income (loss)							(162.6)	(68.3)
Income before income tax (expense) benefit							(162.6)	(68.3)	(26.4)
Income tax (expense) benefit							0.0	0.0	0.0
Net income (loss)							(162.6)	(68.3)
Net loss attributable to noncontrolling interests							0.0	0.0
Net income available to SiriusPoint							$ (162.6)	$ (68.3)	$ (26.4)
Underwriting Ratios:
Loss ratio							76.10%	79.90%	66.80%
Acquisition cost ratio							26.10%	27.80%	33.50%
Other underwriting expenses ratio							7.80%	4.20%	4.10%
Combined ratio							110.00%	111.90%	104.40%
Operating Segments | Services
Segment Reporting Information [Line Items]
Net fee-related income							$ 13.2	$ 0.7
Net realized and unrealized investment gains (losses)							(4.5)
Other revenues							133.7	1.7
Net corporate and other expenses							(120.5)	(1.0)
Net loss attributable to noncontrolling interests							2.3	(0.3)
Net income available to SiriusPoint							11.0	0.4
Eliminations
Segment Reporting Information [Line Items]
Gross premiums written							0.0	0.0
Net premiums written							0.0	0.0
Net premiums earned							0.0	0.0
Loss and loss adjustment expenses incurred, net							(2.6)	0.0
Acquisition costs, net							(67.6)	(0.1)
Other underwriting expenses							0.0	0.0
Net underwriting income (loss)							70.2	0.1
Segment income (loss)							(12.4)	(1.6)
Income before income tax (expense) benefit							(12.4)	(1.6)
Income tax (expense) benefit							0.0	0.0
Net income (loss)							(12.4)	(1.6)
Net loss attributable to noncontrolling interests							0.0	0.0
Net income available to SiriusPoint							(12.4)	(1.6)
Eliminations | Services
Segment Reporting Information [Line Items]
Net fee-related income							(82.6)	(1.7)
Net realized and unrealized investment gains (losses)							0.0
Other revenues							(82.6)	(1.7)
Net corporate and other expenses							0.0	0.0
Net loss attributable to noncontrolling interests							0.0	0.0
Net income available to SiriusPoint							(82.6)	(1.7)
Corporate
Segment Reporting Information [Line Items]
Gross premiums written							(11.8)	28.9	$ 87.6
Net premiums written							(43.5)	28.9	87.6
Net premiums earned							(16.7)	28.1	88.6
Loss and loss adjustment expenses incurred, net							8.9	(0.1)	(4.7)
Acquisition costs, net							3.0	25.4	91.0
Other underwriting expenses							24.1	5.9	9.1
Net underwriting income (loss)							(52.7)	(3.1)	(6.8)
Segment income (loss)							(52.7)	(3.1)
Net realized and unrealized investment gains (losses)							(12.4)	69.2	15.3
Net realized and unrealized investment gains from related party investment funds							304.0	195.0	249.6
Other net investment income							25.4	14.7	17.6
Other revenues							100.1
Net corporate and other expenses							(146.1)	(40.9)	(36.2)
Intangible asset amortization							(5.9)
Interest expense							(34.0)	(8.2)	(8.2)
Foreign exchange gains (losses)							44.0	(5.2)	(3.6)
Income before income tax (expense) benefit							222.4	221.5	227.7
Income tax (expense) benefit							10.7	(8.1)	(0.7)
Net income (loss)							233.1	213.4
Net loss attributable to noncontrolling interests							0.0	0.0
Net income available to SiriusPoint							233.1	213.4	227.0
Segment Measure Reclass
Segment Reporting Information [Line Items]
Segment income (loss)							71.6	1.3
Net realized and unrealized investment gains (losses)							(4.5)	0.0
Other revenues							51.1
Net corporate and other expenses							(120.5)	(1.0)
Income before income tax (expense) benefit							(2.3)	0.3
Income tax (expense) benefit							0.0	0.0
Net income (loss)							(2.3)	0.3
Net loss attributable to noncontrolling interests							2.3	(0.3)
Net income available to SiriusPoint							0.0	0.0
Segment Measure Reclass | Services
Segment Reporting Information [Line Items]
Net fee-related income							69.4	1.0
Net realized and unrealized investment gains (losses)							4.5
Other revenues							(51.1)	0.0
Net corporate and other expenses							120.5	1.0
Net loss attributable to noncontrolling interests							(2.3)	0.3
Net income available to SiriusPoint							71.6	1.3
Reinsurance | Operating Segments
Segment Reporting Information [Line Items]
Gross premiums written							1,350.4	534.1	575.3
Net premiums written							1,124.9	497.3	563.9
Net premiums earned							1,210.9	575.6	606.8
Loss and loss adjustment expenses incurred, net							999.6	459.5	404.3
Acquisition costs, net							302.7	160.4	204.2
Other underwriting expenses							105.5	24.0	24.9
Net underwriting income (loss)							(196.9)	(68.3)	(26.6)
Segment income (loss)							(196.6)	(68.3)
Income before income tax (expense) benefit							$ (196.6)	$ (68.3)	$ (26.6)
Underwriting Ratios:
Loss ratio							82.60%	79.80%	66.60%
Acquisition cost ratio							25.00%	27.90%	33.70%
Other underwriting expenses ratio							8.70%	4.20%	4.10%
Combined ratio							116.30%	111.90%	104.40%
Reinsurance | Operating Segments | Services
Segment Reporting Information [Line Items]
Net fee-related income							$ 0.0	$ 0.0
Net realized and unrealized investment gains (losses)							0.3
Other revenues							0.0	0.0
Net corporate and other expenses							0.0	0.0
Net loss attributable to noncontrolling interests							0.0	0.0
Net income available to SiriusPoint							0.3	0.0
Insurance & Services | Operating Segments
Segment Reporting Information [Line Items]
Gross premiums written							897.9	25.5	$ 5.5
Net premiums written							652.8	16.0	5.5
Net premiums earned							522.8	7.1	4.7
Loss and loss adjustment expenses incurred, net							320.6	5.9	3.9
Acquisition costs, net							149.7	1.4	0.4
Other underwriting expenses							29.2	0.2	0.2
Net underwriting income (loss)							23.3	(0.4)	0.2
Segment income (loss)							34.0	0.0
Income before income tax (expense) benefit							$ 34.0	$ 0.0	$ 0.2
Underwriting Ratios:
Loss ratio							61.30%	83.10%	83.00%
Acquisition cost ratio							28.60%	19.70%	8.50%
Other underwriting expenses ratio							5.60%	2.80%	4.30%
Combined ratio							95.50%	105.60%	95.80%
Insurance & Services | Operating Segments | Services
Segment Reporting Information [Line Items]
Net fee-related income							$ 13.2	$ 0.7
Net realized and unrealized investment gains (losses)							(4.8)
Other revenues							133.7	1.7
Net corporate and other expenses							(120.5)	(1.0)
Net loss attributable to noncontrolling interests							2.3	(0.3)
Net income available to SiriusPoint							$ 10.7	$ 0.4