Marketable Securities - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Investments, Debt and Equity Securities [Abstract]
Equity securities, gross unrealized loss	$ 956
Equity securities, gross unrealized gain			$ 344
Marketable securities, realized gains	317	$ 1,677	1,265
Marketable securities, realized loss	$ 565	$ 1,390	$ 586