Investment and other gains - net - Net Gains on Investments in Equity Securities Still Held (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Investments, Debt and Equity Securities [Abstract]
Net investment (losses) gains recognized on equity securities during the period	$ (264)	$ 1,612	$ 3,393	$ (614)
Less: Net realized gains (losses) on equity securities sold during the period	46	(216)	157	(238)
Net unrealized investment (losses) gains recognized on equity securities still held at June 30	$ (310)	$ 1,828	$ 3,236	$ (376)