Insurance and reinsurance (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Incurred and paid claims	$ 20,425,000	$ 14,838,000	$ 24,380,000
Change in claims outstanding and claims IBNR	3,416,000	11,599,000	5,700,000
Cash collateral pledged	47,494	110,743	6,563
Total collateral posted for reinsurance business	60,100	120,500
Collateral released related to reinsurance business	$ 60,400
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	$ 44,100	106,800
US Treasury Bond Securities
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Total collateral posted for reinsurance business	16,000	13,700
Outward insurance
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Incurred and paid claims	3,285,000	0	0
Change in claims outstanding and claims IBNR	112,000	0	0
Inward Reinsurance
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Incurred and paid claims	17,140,000	14,838,000	24,380,000
Change in claims outstanding and claims IBNR	3,304,000	11,599,000	$ 5,700,000
Collateral Reinsurance Agreement
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Cash collateral pledged	44,100	$ 106,800
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	$ 19,600