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Common Stock Plan (Details) - USD ($)
shares in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Compensation expense related to all stock-based compensation plans $ 9,700,000 $ 13,000,000 $ 9,300,000
Employee Service Share-based Compensation, Aggregate Disclosures [Abstract]      
Employee Service Share-based Compensation, Tax Benefit from Exercise of Stock Options 800,000.0 3,400,000.0 0.0
Payments Related to Tax Withholding for Share-based Compensation $ (5,900,000) $ (9,200,000) $ (6,100,000)
Options and SSARs [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant 2.9