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Retirement Plans - Effects of One-percentage Point Change in Assumed Health Care Cost Trend Rates (Detail) - Other Postretirement Benefits
$ in Millions
12 Months Ended
Dec. 31, 2016
USD ($)
Defined Benefit Plan Disclosure [Line Items]  
Other postretirement benefit obligation, 1% increase $ 5.8
Total other postretirement benefits service and interest cost components, 1% increase 0.8
Other postretirement benefit obligation, 1% decrease (5.4)
Total other postretirement benefits service and interest cost components, 1% decrease $ (0.7)