Fair Value Measurements and Disclosures - Assets and Liabilities Reported at Fair Value (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Fair value adjustment of marketable securities and deferred compensation plan liability
Net gain (loss) on marketable securities	$ 2,662	$ 544	$ (564)	$ 2,225
Fair value, measurements, recurring [Member] | Fair value (Level 1) [Member]
Assets and Liabilities Reported at Fair Value
Marketable securities	$ 23,175		$ 23,175		$ 27,098