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Fair Value Quantitative and Qualitative Disclosures - Summary of Sensitivity Analysis Related to WACC (Detail)
$ in Millions
12 Months Ended
Dec. 31, 2020
USD ($)
Growth Rate At 2.00% [member] | WACC At 97.5%[member]  
Disclosure of fair value measurement of assets and liability [line items]  
Increase (decrease) in fair value measurement due to reasonably possible increase in unobservable input, assets $ 370.9
Growth Rate At 2.00% [member] | WACC At 100%[member]  
Disclosure of fair value measurement of assets and liability [line items]  
Increase (decrease) in fair value measurement due to reasonably possible increase in unobservable input, assets 366.2
Growth Rate At 2.00% [member] | WACC At 102.5%[member]  
Disclosure of fair value measurement of assets and liability [line items]  
Increase (decrease) in fair value measurement due to reasonably possible increase in unobservable input, assets 361.7
Growth Rate At 3.00% [member] | WACC At 97.5%[member]  
Disclosure of fair value measurement of assets and liability [line items]  
Increase (decrease) in fair value measurement due to reasonably possible increase in unobservable input, assets 391.8
Growth Rate At 3.00% [member] | WACC At 100%[member]  
Disclosure of fair value measurement of assets and liability [line items]  
Increase (decrease) in fair value measurement due to reasonably possible increase in unobservable input, assets 386.8
Growth Rate At 3.00% [member] | WACC At 102.5%[member]  
Disclosure of fair value measurement of assets and liability [line items]  
Increase (decrease) in fair value measurement due to reasonably possible increase in unobservable input, assets 381.9
Growth rate At 4.00% [member] | WACC At 97.5%[member]  
Disclosure of fair value measurement of assets and liability [line items]  
Increase (decrease) in fair value measurement due to reasonably possible increase in unobservable input, assets 416.5
Growth rate At 4.00% [member] | WACC At 100%[member]  
Disclosure of fair value measurement of assets and liability [line items]  
Increase (decrease) in fair value measurement due to reasonably possible increase in unobservable input, assets 411.1
Growth rate At 4.00% [member] | WACC At 102.5%[member]  
Disclosure of fair value measurement of assets and liability [line items]  
Increase (decrease) in fair value measurement due to reasonably possible increase in unobservable input, assets $ 405.9