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Stock and Incentive Programs for Employees and Directors - Performance Share Units - Narrative (Detail)
$ in Millions
12 Months Ended
Dec. 31, 2017
USD ($)
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]  
Unrecognized share-based compensation cost $ 14.0
Unrecognized compensation expense, weighted-average period of recognition 2 years
Performance share units  
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]  
Unrecognized share-based compensation cost $ 2.0
Unrecognized compensation expense, weighted-average period of recognition 2 years 1 month 6 days