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Other long-term liabilities - Asset Retirement Obligations (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]    
Opening balance $ 43,291 $ 44,166
Obligation assumed from business acquisition and constructed projects 3,226 225
Retirement activities (443) (5,130)
Accretion 2,148 1,974
Change in cash flow estimates 5,657 2,056
Closing balance $ 53,879 $ 43,291