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Derivative financial instruments (Narrative) (Details) - USD ($)
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Derivative Instruments and Hedging Activities Disclosure [Abstract]    
Derivative cash collateral, obligation to return cash $ 100,000 $ 600,000
Derivative cash collateral, advanced to counterparties 0 $ 0
Cash flow hedge gain (loss) to be reclassified within 12 months $ 66,000,000