Insurance and reinsurance (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Incurred and paid claims	$ 4,581,000	$ 2,570,000	$ 11,817,000	$ 10,498,000
Change in claims outstanding and claims IBNR	3,841,000	1,985,000	3,000,000	5,018,000
Cash collateral pledged	48,996	110,666	48,996	110,666	$ 110,743
Total collateral posted for reinsurance business	61,600		61,600
Collateral released related to reinsurance business			59,000
Cash and Cash Equivalents
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Total collateral posted for reinsurance business	45,600		45,600
US Treasury Bond Securities
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Total collateral posted for reinsurance business	16,000		16,000
Outward insurance
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Incurred and paid claims	831,000	0	2,630,000	0
Change in claims outstanding and claims IBNR	220,000	0	9,000	0
Inward Reinsurance
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Incurred and paid claims	3,750,000	2,570,000	9,187,000	10,498,000
Change in claims outstanding and claims IBNR	3,621,000	$ 1,985,000	2,991,000	$ 5,018,000
Collateral Reinsurance Agreement
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Cash collateral pledged	45,600		45,600		$ 106,800
Collateral Reinsurance Agreement, Released September 30, 2023
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Cash collateral pledged	40,600		40,600
Collateral Reinsurance Agreement, Released March 31, 2024
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Cash collateral pledged	$ 21,000		$ 21,000
Cowen Insurance Co and Cowen Re
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Historical Average Claims Ratios, Period, Maximum			10 years