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Stock-Based Compensation Valuation Assumptions (Details) - Options and SSARs [Member] - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected stock price volatility 41.00% 45.00% 54.00%
Expected dividend yield 0.00% 0.00% 0.00%
Risk-free interest rate 2.10% 1.40% 1.50%
Expected life of options 6 years 6 years 6 years
Weighted average fair value of options and SSARs granted $ 8.02 $ 6.99 $ 8.80