Marketable Securities - Narrative (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]
Marketable securities, realized gain	$ 71	$ 143	$ 339	$ 378
Marketable securities, realized loss	$ 64	$ 22	$ 211	$ 350