Net realized and unrealized gains (losses) - Gross Realized and Unrealized Gains and Losses (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Investments, Debt and Equity Securities [Abstract]
Realized gains	$ 2,615	$ 241	$ 10,695	$ 1,749
Realized losses	(47)	(209)	(2,516)	(1,556)
Net unrealized investment gains (losses) recognized on equity securities still held at end of period	319	(39)	9,637	(6,228)
Investment and other gains (losses) – net	$ 2,887	$ (7)	$ 17,816	$ (6,035)