Investments - Gain in change fair value of equity investments (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Investments [Abstract]
Net (losses) gains recognized on equity investments sold during the period	$ (101)	$ (10)	$ (101)	$ 545
Change in unrealized gains (losses) on equity investments held at end of the period	38	(411)	116	199
(Loss) gain on change in fair value of equity investments	$ (63)	$ (421)	$ 15	$ 744