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Reconciliation of Fair Value Measurements Using Significant Unobservable Inputs (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]    
Plan assets at beginning of year $ 785.6  
Plan assets at end of year 873.8 $ 785.6
Significant Non-observable Inputs (Level 3)    
Defined Benefit Plan Disclosure [Line Items]    
Plan assets at beginning of year 122.0 117.6
Relating to assets still held at the reporting date (2.1) (2.1)
Relating to assets sold during the period 7.8 9.3
Purchases, sales and settlements (29.0) (2.8)
Plan assets at end of year 98.7 122.0
Short-term Investments    
Defined Benefit Plan Disclosure [Line Items]    
Plan assets at beginning of year 28.7  
Plan assets at end of year 15.8 28.7
Short-term Investments | Significant Non-observable Inputs (Level 3)    
Defined Benefit Plan Disclosure [Line Items]    
Plan assets at beginning of year 19.7 12.4
Purchases, sales and settlements (19.7) 7.3
Plan assets at end of year   19.7
Real estate funds | Significant Non-observable Inputs (Level 3)    
Defined Benefit Plan Disclosure [Line Items]    
Plan assets at beginning of year 74.3 70.9
Relating to assets still held at the reporting date 3.5 3.4
Plan assets at end of year 77.8 74.3
Private equity | Significant Non-observable Inputs (Level 3)    
Defined Benefit Plan Disclosure [Line Items]    
Plan assets at beginning of year 28.0 34.3
Relating to assets still held at the reporting date (5.6) (5.5)
Relating to assets sold during the period 7.8 9.3
Purchases, sales and settlements (9.3) (10.1)
Plan assets at end of year $ 20.9 $ 28.0