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Operating Leases (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Leases [Abstract]      
Rental expense for all non-cancelable operating leases $ 17,000 $ 17,600 $ 17,500
Future minimum rentals under operating leases:      
Future minimum operating lease payments due, 2018 15,542    
Future minimum operating lease payments due, 2019 13,593    
Future minimum operating lease payments due, 2020 11,496    
Future minimum operating lease payments due, 2021 8,028    
Future minimum operating lease payments due, 2022 7,178    
Future minimum operating lease payments due, 2023 and thereafter $ 24,675