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Risk Management Activities - Market Risk - Foreign Currency - Additional Information (Detail)
1 Months Ended
Dec. 31, 2018
USD ($)
Oct. 31, 2018
USD ($)
Sep. 30, 2018
USD ($)
Jun. 30, 2018
USD ($)
May 31, 2018
USD ($)
Disclosure Of Financial Risk Management [line items]          
Forwards notional value for the period January 2019 to December 2019 $ 1,290,000,000        
Currency risk [member]          
Disclosure Of Financial Risk Management [line items]          
Exposure to risk relating to financial instruments $ 0        
Average strike rate 0.715 0.7075 0.7182 0.7330 0.7517
Currency risk [member] | Forward contract [member]          
Disclosure Of Financial Risk Management [line items]          
Forwards notional value for the period January 2019 to December 2019 $ 50,000,000 $ 60,000,000 $ 100,000,000 $ 60,000,000 $ 96,000,000
Mark to Market value $ 8,700,000