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Summary of Significant Accounting Policies - Earnings Per Common Share (Details) - USD ($)
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Sep. 30, 2015
Jun. 21, 2013
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]          
Average share price (in dollars per share) $ 90.30        
Exchangeable senior notes $ 562,374,000 $ 604,276,000      
Series A Units          
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]          
Instruments to be settled in cash (at least) $ 101,700,000        
Options          
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]          
Anti-dilutive securities excluded from computation of earnings per common share (in shares) 36,075 45,286 88,552    
2013 Notes          
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]          
Fixed interest rate         2.375%
Shares related to the Notes included in the computation for diluted earnings per share (in shares)   344,430      
2013 Notes | Operating Partnership          
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]          
Shares related to the Notes included in the computation for diluted earnings per share (in shares)     309,730    
2015 Notes          
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]          
Exchangeable senior notes $ 575,000,000        
Fixed interest rate 3.125%     3.125%  
Exchange price (in dollars per share) $ 92.80        
Shares related to the Notes included in the computation for diluted earnings per share (in shares) 0 0 0