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Defined Benefit Plans - Reconciliation of Level Three Measurements (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]    
Beginning balance $ 599.6  
Ending balance 677.2 $ 599.6
Group annuity/insurance contracts (Level 3)    
Defined Benefit Plan Disclosure [Line Items]    
Beginning balance 23.6 23.3
Actual return on assets related to assets still held 0.6 0.3
Ending balance $ 24.2 $ 23.6