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Other assets (Amortization of favorable contacts) (Details) - Intangible assets- Favorable contracts - USD ($)
$ in Millions
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Amortization of favorable contracts      
2020 $ 40.4    
2021 0.0    
2022 0.0    
2023 0.0    
2024 0.0    
Net carrying amount $ 40.4 $ 85.5 $ 130.6