Net realized and unrealized gains - Gross Realized and Unrealized Gains and Losses (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Investments, Debt and Equity Securities [Abstract]
Realized gains	$ 278	$ 1,683	$ 339	$ 3,277
Realized losses	(3,430)	(3,671)	(4,177)	(3,839)
Net unrealized investment gains (losses) recognized on equity securities still held at end of period	2,047	(9,917)	955	(7,258)
Investment and other gains (losses) – net	$ (1,105)	$ (11,905)	$ (2,883)	$ (7,820)