Annual report pursuant to Section 13 and 15(d)

Financial Instruments and Risk Management (Gross Notional Amount of Outstanding Derivatives) (Details)

v3.8.0.1
Financial Instruments and Risk Management (Gross Notional Amount of Outstanding Derivatives) (Details) - Foreign Exchange Forward [Member] - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Designated as Hedging Instrument [Member]    
Derivatives, Fair Value [Line Items]    
Currency forward contracts $ 448,448 $ 206,000
Not Designated as Hedging Instrument [Member]    
Derivatives, Fair Value [Line Items]    
Currency forward contracts $ 231,161 $ 184,940