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, 2020	Dec. 31, 2019	Dec. 31, 2018
Investments, Debt and Equity Securities [Abstract]
Total net investment (losses) gains recognized on equity securities during the period	$ (3,137)	$ 4,825	$ (2,538)
Less: Net realized (losses) gains on equity securities sold during the period	(826)	783	(376)
Net unrealized investment (losses) gains recognized on equity securities still held at December 31	$ (2,311)	$ 4,042	$ (2,162)