Investment income and net realized and unrealized gains - Net Gains (Losses) on Investments in Equity Securities Still Held (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Investments, Debt and Equity Securities [Abstract]
Total net investment (losses) gains recognized on equity securities during the period	$ (2,151)	$ 19,351	$ (3,137)
Less: Net realized gains (losses) on equity securities sold during the period	395	1,631	(826)
Net unrealized investment (losses) gains recognized on equity securities still held at December 31	$ (2,546)	$ 17,720	$ (2,311)