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Employee Stock and Incentive Plans - Share-Based Compensation Expense (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-Based Compensation Expense [Line Items]      
Share-Based Compensation Expense $ 3.1 $ 2.6 $ 2.6
Income Tax Benefit 0.9 1.1 1.1
Capitalized Share-Based Compensation Costs 0.0 0.0 0.0
Performance Shares [Member]      
Share-Based Compensation Expense [Line Items]      
Share-Based Compensation Expense 2.1 1.8 1.8
Unrecognized Compensation Cost $ 2.4    
Weighted-Average Period for Recognition (Years/Months) 1 year 8 months    
Restricted Stock Units [Member]      
Share-Based Compensation Expense [Line Items]      
Share-Based Compensation Expense $ 1.0 $ 0.8 $ 0.8
Unrecognized Compensation Cost $ 1.0    
Weighted-Average Period for Recognition (Years/Months) 1 year 8 months