Investment and other gains - net - Net Gains on Investments in Equity Securities Still Held (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Investments, Debt and Equity Securities [Abstract]
Net investment (losses) gains recognized on equity securities during the period	$ (171)	$ 2,775	$ 3,222	$ 2,161
Less: Net realized gains on equity securities sold during the period	123	598	280	359
Net unrealized investment (losses) gains recognized on equity securities still held at September 30	$ (294)	$ 2,177	$ 2,942	$ 1,802