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Forward Equity and Equity Distribution Program (Details) - USD ($)
$ / shares in Units, shares in Millions, $ in Millions
 1 Months Ended 3 Months Ended 9 Months Ended
Jun. 30, 2016
Sep. 30, 2016
Sep. 30, 2016
Sep. 30, 2015
Class of Stock [Line Items]        
Proceeds from issuance of common stock     $ 163.2 $ 54.0
Forward Equity Agreement        
Class of Stock [Line Items]        
Number of shares available for sale under agreement (in shares) 2.9      
Common stock price (in dollars per share) $ 167.25      
Number of common shares issued (in shares)   0.9 0.9  
Proceeds from issuance of common stock   $ 150.3    
Number of common stock shares unsettled (in shares)   2.0 2.0  
Stock repurchase program, authorized amount   $ 500.0 $ 500.0