Commitments and contingencies - Premiums written by source that individually contributed more than 10% of the total (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Concentration Risk [Line Items]
Gross premiums written	$ 3,409.7	$ 2,236.5	$ 588.5
Gross premiums written | Credit concentration risk
Concentration Risk [Line Items]
Gross premiums written	$ 3,409.7	$ 2,236.5	$ 588.5
% of total	100.00%	100.00%	100.00%
Gross premiums written | Credit concentration risk | Aon Corporation and subsidiaries
Concentration Risk [Line Items]
Gross premiums written	$ 625.5	$ 536.6	$ 189.1
% of total	18.30%	24.00%	32.10%
Gross premiums written | Credit concentration risk | Guy Carpenter & Company and subsidiaries
Concentration Risk [Line Items]
Gross premiums written	$ 441.9	$ 414.1	$ 164.6
% of total	13.00%	18.50%	28.00%
Gross premiums written | Credit concentration risk | Arthur J. Gallagher & Co. and subsidiaries
Concentration Risk [Line Items]
Gross premiums written	$ 216.8	$ 244.2	$ 67.6
% of total	6.40%	10.90%	11.50%
Gross premiums written | Credit concentration risk | Other
Concentration Risk [Line Items]
Gross premiums written	$ 2,125.5	$ 1,041.6	$ 167.2
% of total	62.30%	46.60%	28.40%