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Derivative financial instruments - Narrative (Details)
Jun. 30, 2018
USD ($)
Mar. 29, 2018
contract
Interest rate swaps    
Derivative [Line Items]    
Number of interest rate swap contracts | contract   2
Fixed rate 2.85%  
Interest Rate Swap One    
Derivative [Line Items]    
Notional amount $ 200,000,000  
Interest Rate Swap Two    
Derivative [Line Items]    
Notional amount $ 600,000,000