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Financial Risk Management - Proportions of Variable-rate and Fixed-rate Debt (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Statement Line Items [Line Items]    
Significant borrowings as of Closing Date $ 965,859 $ 840,236
Percentage
Fixed interest rate [member]    
Statement Line Items [Line Items]    
Significant borrowings as of Closing Date $ 946,215 $ 820,600 [1]
Percentage [1] 98.00% 98.00%
Floating interest rate [member]    
Statement Line Items [Line Items]    
Significant borrowings as of Closing Date $ 19,644 $ 19,636
Percentage 2.00% 2.00%
[1] Out of the $946 Million fixed rate borrowings $913 Million are short-term.