Investment and other gains (losses) - Net Gains on Investments in Equity Securities Still Held (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Investments, Debt and Equity Securities [Abstract]
Total net investment losses recognized on equity securities during the period	$ (2,538)
Less: Net realized losses on equity securities sold during the period	(376)
Net unrealized investment losses recognized on equity securities still held	$ (2,162)	$ 0	$ 0