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, 2023	Dec. 31, 2022	Dec. 31, 2021
Investments, Debt and Equity Securities [Abstract]
Total net investment gains (losses) recognized on equity securities during the period	$ 3,044	$ (2,151)	$ 19,351
Less: Net realized gains on equity securities sold during the period	975	395	1,631
Net unrealized investment gains (losses) recognized on equity securities still held at December 31	$ 2,069	$ (2,546)	$ 17,720