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Employee Benefit Plans - Schedule of fair values of defined benefit plan's assets (Detail) - Defined Benefit Pension Plan [Member] - USD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]      
Fair value of defined benefit plan assets $ 558.7 $ 425.6 $ 381.6
Fair Value, Inputs, Level 1 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of defined benefit plan assets 83.5 17.5  
Fair Value, Inputs, Level 2 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of defined benefit plan assets 475.2 408.1  
Short-term Investments [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of defined benefit plan assets 83.5 17.5  
Short-term Investments [Member] | Fair Value, Inputs, Level 1 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of defined benefit plan assets 83.5 17.5  
US Government Debt Securities [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of defined benefit plan assets 46.7 137.2  
US Government Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of defined benefit plan assets 46.7 137.2  
Corporate Debt Securities [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of defined benefit plan assets 322.4 19.7  
Corporate Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of defined benefit plan assets 322.4 19.7  
Collective Investment Funds [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of defined benefit plan assets 106.1 251.2  
Collective Investment Funds [Member] | Fair Value, Inputs, Level 2 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of defined benefit plan assets $ 106.1 $ 251.2