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Financial instruments and fair value disclosures - Interest rate risk (Details) - Interest rate risk - Floating rate
$ in Millions
12 Months Ended
Dec. 31, 2017
USD ($)
Financial instruments and fair value disclosures  
Percentage of reasonably possible increase in risk assumption 0.50%
Effect on net earnings $ (4.3)
Percentage of reasonably possible decrease in risk assumption 0.50%
Effect on net earnings $ 4.3