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Share-Based Compensation (PBRSU Weighted Average Assumptions) (Details) - PBRSUs
6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Expected dividend yield 0.00% 0.00%
Risk-free interest rate 2.44% 2.38%
Expected volatility 28.24% 23.63%
Average total stockholder return 17.15% 6.11%