Net realized and unrealized gains - Investment Gains and Losses recognized related to Investments in Equity Securities (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]
Net investment gains (losses) recognized on equity securities during the period	$ 1,988	$ (9,366)	$ 232	$ (6,795)
Less: Net realized (losses) gains on equity securities sold during the period	(59)	551	(723)	463
Net unrealized investment gains (losses) recognized on equity securities still held at end of period	$ 2,047	$ (9,917)	$ 955	$ (7,258)