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Derivative financial instruments - Interest Rate Swaps (Details) - Interest rate swaps - USD ($)
$ in Millions
Mar. 12, 2019
Dec. 31, 2018
Dec. 31, 2016
Not Designated As Hedging Instrument      
Disclosure of detailed information about financial instruments [line items]      
Notional amount of interest rate swaps     $ 100
Derivative fixed interest rate     1.04%
Swap contract      
Disclosure of detailed information about financial instruments [line items]      
Financial assets, at fair value   $ 1  
Entering into significant commitments or contingent liabilities | Not Designated As Hedging Instrument      
Disclosure of detailed information about financial instruments [line items]      
Notional amount of interest rate swaps $ 125    
Bottom of range | Entering into significant commitments or contingent liabilities | Not Designated As Hedging Instrument      
Disclosure of detailed information about financial instruments [line items]      
Derivative fixed interest rate 2.36%    
Top of range | Entering into significant commitments or contingent liabilities | Not Designated As Hedging Instrument      
Disclosure of detailed information about financial instruments [line items]      
Derivative fixed interest rate 2.67%