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