Significant accounting policies - Summary of the retrospective impact of change in accounting policy (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
Condensed consolidated balance sheets
Insurance and reinsurance balances receivable, net	$ 1,708.2						$ 1,708.2	$ 441.9
Deferred acquisition costs and value of business acquired, net	218.8						218.8	68.6
Unearned premiums ceded	242.8						242.8	20.5
Total assets	10,618.3						10,618.3	3,535.2
Reinsurance balances payable	688.3						688.3	78.1
Unearned premium reserves	1,198.4						1,198.4	284.8
Total liabilities	8,115.0						8,115.0	1,969.9
Shareholders’ equity attributable to SiriusPoint shareholders	2,503.7						2,503.7	1,563.9
Condensed consolidated statements of income
Gross premiums written							2,236.5	588.5	$ 668.4
Gross premiums ceded							(502.3)	(46.3)	(11.4)
Net premiums written							1,734.2	542.2	657.0
Change in net unearned premium reserves								68.6	43.1
Net premiums earned	519.9	$ 499.6	$ 452.3	$ 245.2	$ 697.5	$ 1,197.1	1,717.0	610.8	700.1
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 available to SiriusPoint common shareholders	$ (140.3)	$ (48.0)	$ 64.5	$ 168.4	$ 232.9	$ 184.9	44.6	143.5	200.6
Condensed consolidated statement of cash flows
Insurance and reinsurance balances receivable, net							(48.3)	38.6	20.4
Deferred acquisition costs and value of business acquired, net							(2.3)	23.6	24.8
Unearned premiums ceded							(33.3)	(17.3)	(1.5)
Unearned premium reserves							13.6	(51.3)	(41.6)
Reinsurance balances payable							223.0	1.8	11.5
Net cash provided by operating activities							$ 1.6	73.3	141.1
As previously reported
Condensed consolidated balance sheets
Insurance and reinsurance balances receivable, net								559.4
Deferred acquisition costs and value of business acquired, net								134.3
Unearned premiums ceded								27.7
Total assets								3,725.6
Reinsurance balances payable								80.4
Unearned premium reserves								472.9
Total liabilities								2,160.3
Shareholders’ equity attributable to SiriusPoint shareholders								1,563.9
Condensed consolidated statements of income
Gross premiums written								588.0	631.8
Gross premiums ceded								(39.7)	(9.3)
Net premiums written								548.3	622.5
Change in net unearned premium reserves								62.5	77.6
Net premiums earned								610.8	700.1
Net income available to SiriusPoint common shareholders								143.5	200.6
Net income available to SiriusPoint common shareholders								143.5	200.6
Condensed consolidated statement of cash flows
Insurance and reinsurance balances receivable, net								38.8	30.0
Deferred acquisition costs and value of business acquired, net								20.4	49.1
Unearned premiums ceded								(10.7)	0.6
Unearned premium reserves								(51.8)	(78.1)
Reinsurance balances payable								(1.3)	12.0
Net cash provided by operating activities								73.3	141.1
Adjustment
Condensed consolidated balance sheets
Insurance and reinsurance balances receivable, net								(117.5)
Deferred acquisition costs and value of business acquired, net								(65.7)
Unearned premiums ceded								(7.2)
Total assets								(190.4)
Reinsurance balances payable								(2.3)
Unearned premium reserves								(188.1)
Total liabilities								(190.4)
Shareholders’ equity attributable to SiriusPoint shareholders								0.0
Condensed consolidated statements of income
Gross premiums written								0.5	36.6
Gross premiums ceded								(6.6)	(2.1)
Net premiums written								(6.1)	34.5
Change in net unearned premium reserves								6.1	(34.5)
Net premiums earned								0.0	0.0
Net income available to SiriusPoint common shareholders								0.0	0.0
Net income available to SiriusPoint common shareholders								0.0	0.0
Condensed consolidated statement of cash flows
Insurance and reinsurance balances receivable, net								(0.2)	(9.6)
Deferred acquisition costs and value of business acquired, net								3.2	(24.3)
Unearned premiums ceded								(6.6)	(2.1)
Unearned premium reserves								0.5	36.5
Reinsurance balances payable								3.1	(0.5)
Net cash provided by operating activities								$ 0.0	$ 0.0