Investment income and net realized and unrealized gains - Income from Investments and Net Realized and Unrealized Gains (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Debt Securities, Available-for-sale [Line Items]
Investment income	$ 45,135	$ 22,421	$ 16,855
Net realized and unrealized (losses) gains:
Realized gains	3,407	6,212	13,015
Realized losses	(5,510)	(5,142)	(6,414)
Net unrealized investment gains (losses) recognized on equity securities still held	2,069	(2,546)	17,720
Investment and other gains (losses) – net	(34)	(1,476)	24,321
Debt securities
Debt Securities, Available-for-sale [Line Items]
Investment income	16,274	14,483	13,313
Equity securities, short-term investments, cash equivalents and other
Debt Securities, Available-for-sale [Line Items]
Investment income	$ 28,861	$ 7,938	$ 3,542