Fair Value Measurements and Disclosures (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Fair value adjustment of marketable securities and deferred compensation plan liability
Net gain on marketable securities	$ 271	$ 536	$ 289	$ 1,719
Fair value, measurements, recurring [Member] | Fair value (Level 1) [Member]
Assets and Liabilities Reported at Fair Value
Marketable securities	$ 23,353		$ 23,353		$ 20,674