Net realized and unrealized gains (losses) - Gross Realized and Unrealized Gains and Losses (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Investments, Debt and Equity Securities [Abstract]
Realized gains	$ 7,909	$ 1,355	$ 8,080	$ 1,508
Realized losses	0	(657)	(2,469)	(1,347)
Net unrealized investment gains (losses) recognized on equity securities still held at end of period	3,745	4,366	9,318	(6,188)
Investment and other gains (losses) – net	$ 11,654	$ 5,064	$ 14,929	$ (6,027)