Commitments and contingencies - Narrative (Details)			12 Months Ended
	Jul. 02, 2021 USD ($)	Sep. 16, 2020 USD ($)	Dec. 31, 2021 USD ($) contract	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Related Party Transaction [Line Items]
Operating lease expense			$ 10,500,000	$ 900,000	$ 900,000
Operating lease right-of-use assets			$ 27,400,000	$ 800,000
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible Enumeration]			Other assets	Other assets
Operating lease liabilities			$ 32,500,000	$ 800,000
Operating Lease, Liability, Statement of Financial Position [Extensible Enumeration]			Accounts payable, accrued expenses and other liabilities	Accounts payable, accrued expenses and other liabilities
Number of lease contracts not yet commenced | contract			1
Future minimum rental commitments for lease not yet commenced			$ 11,400,000
Affiliated entity
Related Party Transaction [Line Items]
Interest rate	8.00%	8.00%
Amount drawn			0
Loans receivable	$ 11,500,000
Loan receivable, maximum increase	$ 5,000,000
Contributions to Central Fund | Lloyd's Central Fund
Related Party Transaction [Line Items]
Commitment amount			$ 600,000
Percentage of gross premiums written			0.35%
Maximum additional contribution, percentage			3.00%
Commitments to Extend Credit | Affiliated entity
Related Party Transaction [Line Items]
Commitment amount		$ 18,000,000