Net realized and unrealized gains - Gross Realized and Unrealized Gains and Losses (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Investments, Debt and Equity Securities [Abstract]
Realized gains	$ 900	$ 183	$ 1,239	$ 3,460
Realized losses	(307)	(65)	(4,484)	(3,904)
Net unrealized investment losses recognized on equity securities still held at end of period	(2,539)	(6,492)	(1,584)	(13,750)
Investment and other gains (losses) – net	$ (1,946)	$ (6,374)	$ (4,829)	$ (14,194)