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Derivatives - Foreign Currency Derivative Contracts (Details) - Not Designated as Hedging Instrument
$ in Thousands
Dec. 31, 2017
USD ($)
$ / €
$ / CAD
$ / £
$ / NOK
Dec. 31, 2016
USD ($)
$ / €
$ / CAD
$ / £
$ / NOK
Canadian dollar | Foreign currency contracts    
Derivative [Line Items]    
Notional amount $ 6,226 $ 4,553
Contractual exchange rate (percent) | $ / CAD 1.2850 1.3179
Euro | Foreign currency contracts    
Derivative [Line Items]    
Notional amount $ 5,326 $ 4,753
Contractual exchange rate (percent) | $ / € 1.1836 1.0563
Euro | Foreign currency contracts, settlement date January 13, 2017    
Derivative [Line Items]    
Notional amount   $ 2,558
Contractual exchange rate (percent) | $ / €   1.0659
Norwegian krone | Foreign currency contracts    
Derivative [Line Items]    
Notional amount $ 6,212 $ 3,643
Contractual exchange rate (percent) | $ / NOK 8.3704 8.5101
Pound sterling | Foreign currency contracts    
Derivative [Line Items]    
Notional amount $ 6,039 $ 3,908
Contractual exchange rate (percent) | $ / £ 1.3419 1.2607