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Risk Management Activities - Market Risk - Interest Rate - Additional Information (Detail) - USD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Disclosure of Financial Risk Management [line items]    
Borrowings $ 1,526.9 $ 1,845.8
Interest Bearing Borrowings [Member] | LIBOR [Member]    
Disclosure of Financial Risk Management [line items]    
Borrowings 333.5 83.5
Change in Bank Bill Swap Bid Rate [member] | Interest Bearing Borrowings [Member] | Floating Interest Rate [member]    
Disclosure of Financial Risk Management [line items]    
Borrowings exposed to interest rate fluctuations 200.0 168.5
Sensitivity to interest rates [member] | Interest Bearing Borrowings [Member]    
Disclosure of Financial Risk Management [line items]    
Borrowings exposed to interest rate fluctuations $ 533.5 $ 252.0