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Fair Value Measurements and Disclosures - Assets and Liabilities Reported at Fair Value (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Dec. 31, 2018
Fair value adjustment of marketable securities and deferred compensation plan liability          
Net gain on marketable securities $ 544 $ 422 $ 2,225 $ 18  
Fair value, measurements, recurring [Member] | Fair value (Level 1) [Member]          
Assets and Liabilities Reported at Fair Value          
Marketable securities $ 25,203   $ 25,203   $ 21,779