Net realized and unrealized gains - Investment Gains and Losses recognized related to Investments in Equity Securities (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]
Net investment losses recognized on equity securities during the period	$ (1,738)	$ (6,489)	$ (1,505)	$ (13,284)
Less: Net realized gains on equity securities sold during the period	801	3	79	466
Net unrealized investment losses recognized on equity securities still held at end of period	$ (2,539)	$ (6,492)	$ (1,584)	$ (13,750)