Annual report pursuant to Section 13 and 15(d)

Fair Value Measurements (Details)

v3.10.0.1
Fair Value Measurements (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Assets and liabilities measured at fair value on a recurring basis    
Marketable securities in deferred compensation plan $ 3,868 $ 4,616
Deferred Compensation, Excluding Share-based Payments and Retirement Benefits [Member] | Trustees and Management [Member]    
Assets and liabilities measured at fair value on a recurring basis    
Maximum percentage of participants' compensation which is deferrable (as a percent) 100.00%  
Marketable securities in deferred compensation plan $ 3,900 $ 4,600