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, 2021	Dec. 31, 2020	Dec. 31, 2019
Concentration Risk [Line Items]
Gross premiums written	$ 2,236.5	$ 588.5	$ 668.4
Gross premiums written | Credit concentration risk
Concentration Risk [Line Items]
Gross premiums written	$ 2,236.5	$ 588.5	$ 668.4
% 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	$ 536.6	$ 189.1	$ 197.1
% of total	24.00%	32.10%	29.50%
Gross premiums written | Credit concentration risk | Guy Carpenter & Company and subsidiaries
Concentration Risk [Line Items]
Gross premiums written	$ 414.1	$ 164.6	$ 144.6
% of total	18.50%	28.00%	21.60%
Gross premiums written | Credit concentration risk | Arthur J. Gallagher & Co. and subsidiaries
Concentration Risk [Line Items]
Gross premiums written	$ 244.2	$ 67.6	$ 140.2
% of total	10.90%	11.50%	21.00%
Gross premiums written | Credit concentration risk | Other
Concentration Risk [Line Items]
Gross premiums written	$ 1,041.6	$ 167.2	$ 186.5
% of total	46.60%	28.40%	27.90%