Insurance and reinsurance (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021	Dec. 31, 2021
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Incurred and paid claims	$ 4,078,000	$ 4,648,000
Change in claims outstanding and claims IBNR	(535,000)	(695,000)
Total collateral posted for reinsurance business	61,400		$ 120,500
Collateral released related to reinsurance business	1,400
Cash collateral pledged	$ 52,865	110,303	47,494
Cowen Insurance Co and Cowen Re
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Historical Average Claims Ratios, Period, Maximum	10 years
Cash and Cash Equivalents
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Total collateral posted for reinsurance business	$ 49,400		106,800
US Treasury Bond Securities
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Total collateral posted for reinsurance business	12,000		13,700
Outward insurance
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Incurred and paid claims	1,067,000	744,000
Change in claims outstanding and claims IBNR	54,000	(136,000)
Inward Reinsurance
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Incurred and paid claims	3,011,000	3,904,000
Change in claims outstanding and claims IBNR	(589,000)	$ (559,000)
Collateral Reinsurance Agreement
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Cash collateral pledged	49,500		$ 44,100
Collateral Reinsurance Agreement, Released September 30, 2023
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Cash collateral pledged	40,500
Collateral Reinsurance Agreement, Released March 31, 2024
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Cash collateral pledged	$ 20,900