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Intangibles (Lease premium and related accumulated amortization) (Details) - Lease Premium - USD ($)
$ in Thousands
Jun. 30, 2019
Dec. 31, 2018
Finite-Lived Intangible Assets [Line Items]    
Gross carrying amount $ 67,606 $ 73,300
Accumulated amortization (47,851) (48,356)
Finite-lived intangible assets $ 19,755 $ 24,944