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Share-Based Compensation (PBRSU Weighted Average Assumptions) (Details) - PBRSUs
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Expected dividend yield 0.00% 0.00%
Risk-free interest rate 2.38% 1.47%
Expected volatility 23.63% 27.83%
Average total stockholder return 6.11% 1.46%