Annual and transition report of foreign private issuers pursuant to Section 13 or 15(d)

Fair value measurements (Financial assets and liabilities measured at fair value on recurring basis) (Details)

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Fair value measurements (Financial assets and liabilities measured at fair value on recurring basis) (Details) - Fair Value, Measurements, Recurring [Member] - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative assets $ 48,896 $ 37,187
Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative assets 0 0
Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative assets 48,896 37,187
Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative assets $ 0 $ 0