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Defined Benefit Plans - Reconciliation of Level Three Measurements (Detail) - Group annuity/insurance contracts (Level 3) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]    
Beginning balance $ 22.8 $ 22.3
Actual return on assets related to assets still held 0.5 0.5
Ending balance $ 23.3 $ 22.8