Transactions with related parties (Details) - USD ($)		3 Months Ended		9 Months Ended
	Mar. 27, 2020	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Transactions with related parties
Gross written premiums		$ 356,000,000.0	$ 413,700,000	$ 1,496,200,000	$ 1,523,100,000
Receivables due from affiliates		22,000,000.0		22,000,000.0		$ 16,100,000
Payables due to affiliates		900,000		900,000		$ 900,000
Payments for litigation				2,200,000
Insurance and MGU affiliates, or their subsidiaries
Transactions with related parties
Gross written premiums		$ 16,000,000.0	$ 21,700,000	$ 84,100,000	$ 70,700,000
Holders of series B preference shares | Expense reimbursement
Transactions with related parties
Amount of transaction	$ 250,000
Holders of series B preference shares | Expense reimbursement requiring written consent
Transactions with related parties
Amount of transaction	1,000,000
Holders of series B preference shares | Other professional fees reimbursement
Transactions with related parties
Amount of transaction	$ 500,000