Allowance for expected credit losses - Summary of allowance for expected credit losses (Details) - USD ($) $ in Millions	Sep. 30, 2023	Jun. 30, 2023	Dec. 31, 2022	Sep. 30, 2022	Dec. 31, 2021
Receivables [Abstract]
Insurance and reinsurance balances receivable, net	$ 2,057.6	$ 2,219.6	$ 1,876.9
Loss and loss adjustment expenses recoverable, net	2,314.2	$ 2,276.6	1,376.2	$ 1,309.2	$ 1,215.3
Other assets	78.5		52.4
Total assets in scope	$ 4,450.3		$ 3,305.5