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Derivatives - Foreign Currency Derivative Contracts Outstanding (Details) - Not Designated as Hedging Instrument - Foreign currency contracts
$ in Thousands
Mar. 31, 2019
USD ($)
$ / £
$ / €
$ / kr
$ / $
Dec. 31, 2018
USD ($)
$ / £
$ / €
$ / kr
$ / $
Canadian dollar    
Derivative [Line Items]    
Notional Amount $ 1,349 $ 2,248
Contractual Exchange Rate | $ / $ 1.3342 1.3343
Euro    
Derivative [Line Items]    
Notional Amount $ 7,959 $ 6,967
Contractual Exchange Rate | $ / € 1.1370 1.1421
Norwegian krone    
Derivative [Line Items]    
Notional Amount $ 9,333 $ 7,713
Contractual Exchange Rate | $ / kr 8.5721 8.5566
Pound sterling    
Derivative [Line Items]    
Notional Amount $ 19,300 $ 16,452
Contractual Exchange Rate | $ / £ 1.3310 1.2655