Investment and other (losses) gains - net - Net Gains on Investments in Equity Securities Still Held (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Total net investment losses recognized on equity securities during the period	$ (2,226)
Less: Net realized losses on equity securities sold during the period	(23)
Net unrealized investment losses recognized on equity securities still held	$ (2,203)	$ 0