Transactions with related parties (Details) - USD ($)			3 Months Ended		6 Months Ended
	Aug. 10, 2020	Mar. 27, 2020	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Transactions with related parties
Gross written premiums			$ 322,600,000	$ 487,100,000	$ 1,140,200,000	$ 1,109,400,000
Receivables due from affiliates			31,600,000		31,600,000		$ 16,100,000
Payables due to affiliates			900,000		900,000		$ 900,000
Insurance and MGU affiliates, or their subsidiaries
Transactions with related parties
Gross written premiums			$ 26,800,000	$ 24,900,000	$ 68,100,000	$ 49,000,000
Expense Reimbursement | Holders of series B preference shares
Transactions with related parties
Amount of transaction		$ 250,000
Expense Reimbursement Requiring Written Consent | Holders of series B preference shares
Transactions with related parties
Amount of transaction		1,000,000
Other Professional Fees Reimbursement | Holders of series B preference shares
Transactions with related parties
Amount of transaction		$ 500,000
Subsequent event
Transactions with related parties
Payments for litigation	$ 2,000,000