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Employee Retirement Plans - Defined Benefit Plans - Additional Information (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Sep. 01, 2015
Defined Benefit Plan Disclosure [Line Items]          
Effect of one-percentage-point decrease in the aggregate of service and interest cost components of annual expense   $ 0.1      
Effect of one-percentage-point increase in the aggregate of service and interest cost components of annual expense   $ 0.1      
Pension Benefits [Member]          
Defined Benefit Plan Disclosure [Line Items]          
Settlement loss     $ 5.8    
Discount rate   4.34% 3.98% 4.81%  
Spot rate for interest cost   3.97% 4.81% 4.16%  
Expected contributions in the next fiscal year   $ 2.0      
OPEB Benefits [Member]          
Defined Benefit Plan Disclosure [Line Items]          
Changes in OPEB benefit obligation   $ (29.8)      
Discount rate   3.61% 3.90% 4.65% 3.59%
Increase (decrease) in service cost due to remeasurement   $ (0.2)      
Increase (decrease) in interest cost due to remeasurement   $ (0.2)      
Defined benefit plan weighted average rates, service cost   3.59%      
Defined benefit plan weighted average rates, interest cost   3.59%      
Spot rate for interest cost   3.60% 4.65% 4.39%  
Expected contributions in the next fiscal year   $ 7.3      
OPEB Benefits [Member] | Minimum [Member]          
Defined Benefit Plan Disclosure [Line Items]          
Spot rate for service cost   0.65%      
Spot rate for interest cost   0.65%      
OPEB Benefits [Member] | Maximum [Member]          
Defined Benefit Plan Disclosure [Line Items]          
Spot rate for service cost   5.01%      
Spot rate for interest cost   5.01%      
OPEB Benefits [Member] | Pension Benefits [Member] | Estimated [Member]          
Defined Benefit Plan Disclosure [Line Items]          
Defined benefit plan weighted average rates, service cost 4.37%        
Defined benefit plan weighted average rates, interest cost 4.37%        
Estimated increase (decrease) in service cost $ (7.0)        
Estimated increase (decrease) in interest cost $ (7.0)        
OPEB Benefits [Member] | Pension Benefits [Member] | Estimated [Member] | Minimum [Member]          
Defined Benefit Plan Disclosure [Line Items]          
Spot rate for service cost 0.92%        
Spot rate for interest cost 0.92%        
OPEB Benefits [Member] | Pension Benefits [Member] | Estimated [Member] | Maximum [Member]          
Defined Benefit Plan Disclosure [Line Items]          
Spot rate for service cost 4.97%        
Spot rate for interest cost 4.97%