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Financial Risk Management - Proportions of Variable-rate and Fixed-rate Debt (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Statement Line Items [Line Items]    
Significant borrowings as of closing date $ 539,007 $ 965,859
Fixed interest rate [member]    
Statement Line Items [Line Items]    
Significant borrowings as of closing date $ 520,471 $ 946,215 [1]
Percentage [1] 97.00% 98.00%
Floating interest rate [member]    
Statement Line Items [Line Items]    
Significant borrowings as of closing date $ 18,536 $ 19,644
Percentage 3.00% 2.00%
[1] Out of the $520 million fixed rate borrowings $493 million are short-term.