Net realized and unrealized (losses) gains - Gross Realized and Unrealized Gains and Losses (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Investments, Debt and Equity Securities [Abstract]
Realized gains	$ 1,355	$ 791	$ 1,508	$ 953
Realized losses	(657)	(59)	(1,347)	(363)
Net unrealized investment gains (losses) recognized on equity securities still held at June 30	4,366	(310)	(6,188)	3,236
Investment and other gains (losses) – net	5,064	422	(6,027)	$ 3,826
Equity method investment, realized gain	1,100	$ 700
Debt securities, realized losses	$ 600		$ 1,200