Investment and other gains - net - Gross Realized Investment and Other Gains and Losses (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]
Investment and other gains	$ 791	$ 603	$ 953	$ 1,166
Investment and other losses	(59)	(38)	(363)	(68)
Net unrealized investment (losses) gains recognized on equity securities still held at June 30	(310)	1,828	3,236	(376)
Investment and other gains (losses) – net	$ 422	$ 2,393	$ 3,826	$ 722