Insurance and reinsurance (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Incurred and paid claims	$ 4,648,000	$ 4,235,000
Change in claims outstanding and claims IBNR	695,000	17,293,000
Cash collateral pledged	110,303	7,538	$ 110,743
Total collateral posted for reinsurance business	120,500
Outward insurance
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Incurred and paid claims	744,000	0
Change in claims outstanding and claims IBNR	136,000	0
Inward Reinsurance
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Incurred and paid claims	3,904,000	4,235,000
Change in claims outstanding and claims IBNR	559,000	$ 17,293,000
Collateral Reinsurance Agreement
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Cash collateral pledged	106,800		$ 106,800
Collateral Reinsurance Agreement, Released September 30, 2023
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Cash collateral pledged	101,200
Collateral Reinsurance Agreement, Released March 31, 2024
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Cash collateral pledged	$ 5,600
Hollenfels
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	$ 106,800
US Treasury Bond Securities [Member]
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Total collateral posted for reinsurance business	$ 13,700