Insurance and reinsurance (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Incurred and paid claims	$ 33,111,000	$ 20,425,000	$ 14,838,000
Change in claims outstanding and claims IBNR	(55,270,000)	3,416,000	11,599,000
Reinsurance Recoverable for Paid Claims and Claims Adjustments	66,303	9,072
Deferred Policy Acquisition Cost	4,610	5,672
Cash advances held by cedants	2,684	3,523
Reinsurance business receivable	60,189	11,806	0
Insurance and reinsurance assets	133,786	30,073
Insurance and reinsurance assets	27,030	3,837
Unearned Premiums	27,294	23,241
Loss reserves and claims incurred but not reported	263,223	44,191
Insurance and reinsurance liabilities	317,547	71,269
Cash collateral pledged	$ 122,687	47,494	110,743
Cowen Insurance & Cowen Re
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Historical Average Claims Ratios, Period, Maximum	10 years
Total collateral posted for reinsurance business	$ 53,000	60,100
Collateral released related to reinsurance business	7,100
Cowen Insurance & Cowen Re | Collateral Reinsurance Agreement, Released September 30, 2023
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Cash collateral pledged	40,500
Cowen Insurance & Cowen Re | Collateral Reinsurance Agreement, Released March 31, 2024
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Cash collateral pledged	12,500
Cowen Insurance & Cowen Re | Cash and Cash Equivalents
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Total collateral posted for reinsurance business	43,000	44,100
Cowen Insurance & Cowen Re | US Treasury Bond Securities
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Total collateral posted for reinsurance business	10,100	16,000
Kelvin Re Limited
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Total collateral posted for reinsurance business	76,600
Kelvin Re Limited | Cash and Cash Equivalents
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Total collateral posted for reinsurance business	52,200
Kelvin Re Limited | Letter of Credit
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Total collateral posted for reinsurance business	24,400
Outward insurance
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Incurred and paid claims	4,496,000	3,285,000	0
Change in claims outstanding and claims IBNR	533,000	112,000	0
Inward Reinsurance
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Incurred and paid claims	28,615,000	17,140,000	14,838,000
Change in claims outstanding and claims IBNR	$ (55,803,000)	$ 3,304,000	$ 11,599,000