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Risk Management Activities - Summary of the Exposure to Credit Risk for Trade Receivables by Geographic Region (Parenthetical) (Detail) - USD ($)
$ in Millions
Dec. 31, 2019
Dec. 31, 2018
Jan. 01, 2018
Dec. 31, 2017
Dec. 31, 2016
 [1]
Disclosure Of Credit Risk Exposure For Trade Receivables [line items]          
Cash and cash equivalents $ 515.0 $ 219.7 [1] $ 393.8 $ 393.8 [1] $ 471.0
Environmental trust funds 69.5 60.8 [1] $ 55.5 $ 55.5  
Banks and financial institutions [member]          
Disclosure Of Credit Risk Exposure For Trade Receivables [line items]          
Environmental trust funds $ 69.5 $ 60.8      
[1] Refer note 42.