Marketable Securities - Narrative (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]
Marketable securities, realized gain	$ 174	$ 22	$ 268	$ 235
Marketable securities, realized loss	$ 0	$ 194	$ 147	$ 328