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Financial instruments - Carrying values of financial instruments (Details) - USD ($)
$ in Thousands
Dec. 31, 2019
Dec. 31, 2018
Disclosure of fair value measurement of liabilities [line items]    
Carrying value $ 2,370,617 $ 2,087,972
Long-term debt excluding deferred financing fees | Level 2    
Disclosure of fair value measurement of liabilities [line items]    
Carrying value 1,786,025 1,472,117
Fair value $ 1,831,292 $ 1,442,046