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Stock-Based Compensation - Valuation Assumptions (Details)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Risk free interest rate 1.47% 1.09% 1.13%
Total dividend payments over the measurement period per share 2.30% 2.40% 1.50%
Outperformance Program      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Risk free interest rate 1.63%    
Total dividend payments over the measurement period per share 3.20%    
The Company      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected volatility rate 24.00% 24.00% 22.00%
The Company | Outperformance Program      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected volatility rate 23.00%    
REIT Index      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected volatility rate 17.00% 17.00% 22.00%
REIT Index | Outperformance Program      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected volatility rate 18.00%