Related party transactions - Narrative (Details) - USD ($)	3 Months Ended			6 Months Ended	9 Months Ended
	Sep. 30, 2023	Jun. 30, 2023	Sep. 30, 2022	Jun. 30, 2023	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2022	Jun. 30, 2022	Feb. 23, 2022	Mar. 01, 2021	Feb. 26, 2021	Jan. 01, 2019
Related Party Transaction [Line Items]
Gross premiums written	$ 758,800,000	$ 842,500,000	$ 843,800,000	$ 1,953,000,000	$ 2,711,800,000	$ 2,666,100,000
Due to related party	2,057,600,000	$ 2,219,600,000		$ 2,219,600,000	2,057,600,000		$ 1,876,900,000
Due from related party	1,733,400,000				1,733,400,000		813,600,000
Management fees | Third Point LLC, 2022 LPA
Related Party Transaction [Line Items]
Fee percentage, annual									1.25%
Management fees | Third Point LLC, 2020 LPA
Related Party Transaction [Line Items]
Fee percentage, annual											1.25%
Management fees | Third Point LLC, 2022 Venture II LPA
Related Party Transaction [Line Items]
Fee percentage, annual								0.75%
Management fees | Third Point LLC, TPIPS IMA
Related Party Transaction [Line Items]
Fee percentage, annual											0.06%
Management fees | Third Point LLC, 2022 IMA
Related Party Transaction [Line Items]
Fee percentage, annual									0.50%
Advisory fees | Third Point LLC, 2022 IMA
Related Party Transaction [Line Items]
Fee amount, annual									$ 1,500,000
Performance fees | TP GP
Related Party Transaction [Line Items]
Fee percentage, annual												20.00%
Performance fees | TP Venture GP
Related Party Transaction [Line Items]
Fee percentage, annual										20.00%
Performance fees | TP Venture GP II
Related Party Transaction [Line Items]
Fee percentage, annual								20.00%
Incentive fees | Third Point LLC, 2022 IMA
Related Party Transaction [Line Items]
Fee percentage, annual									15.00%
Related Party
Related Party Transaction [Line Items]
Gross premiums written	78,700,000		$ 144,900,000		226,000,000	$ 262,900,000
Due to related party	31,300,000				31,300,000		224,600,000
Due from related party	$ 0				$ 0		$ 100,000
Related Party | Management fees | Third Point LLC, 2022 IMA
Related Party Transaction [Line Items]
Fee percentage, annual									0.50%
Related Party | Advisory fees | Third Point LLC, 2022 IMA
Related Party Transaction [Line Items]
Fee amount, annual									$ 1,500,000
Related Party | Incentive fees | Third Point LLC, 2022 IMA
Related Party Transaction [Line Items]
Fee percentage, annual									15.00%