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Other long-term liabilities - Asset Retirement Obligations (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]    
Opening Balance $ 44,166 $ 18,486
Obligation assumed from business acquisition and constructed projects 225 28,267
Retirement activities (5,130) (2,811)
Accretion 1,974 1,981
Change in cash flow estimates 2,056 (1,757)
Closing Balance $ 43,291 $ 44,166