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Stock-Based Compensation (Assumptions for PSU Awards) (Details) - Performance Shares [Member]
12 Months Ended
Dec. 31, 2017
USD ($)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Weighted-average risk-free interest rate 1.59%
Weighted-average expected life 3 years
Weighted-average expected volatility 36.50%
Weighted-average expect dividends $ 0
Forfeiture rate 0.00%