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Share-Based Compensation - Assumptions Used in the Determination of Grant-Date Fair Value of Awards (Details) - TSR Stock Awards - $ / shares
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Grant date market value of a common share (dollars per share) $ 40.95 $ 44.07 $ 38.73
Risk-free interest rate 2.32% 1.57% 1.15%
Expected dividend yield 3.52% 3.27% 3.41%
Expected volatility 18.02% 21.33% 21.24%
Derived vesting period of TSR Restricted Stock and TSR LTIP I units 3 years 5 months 3 years 5 months 3 years 5 months
Weighted average expected term of options 5 years 7 months 5 years 9 months 5 years 9 months