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Commitments and Contingencies Disclosure (Narrative) (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Site Contingency [Line Items]        
Accrual for estimated environmental remediation costs $ 8,925 $ 9,083 $ 4,979 $ 12,770
Environmental liability, next 12 months 925      
Environmental liability thereafter $ 8,000      
Remediation activity monitoring period 15 years      
Minimum        
Site Contingency [Line Items]        
Remediation period 2 years      
Maximum        
Site Contingency [Line Items]        
Remediation period 3 years