XML 89 R66.htm IDEA: XBRL DOCUMENT v3.8.0.1
Forward Equity and Equity Distribution Program (Details) - USD ($)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Class of Stock        
Proceeds from issuance of common stock $ 41,900,000 $ 465,800,000 $ 57,800,000  
Common stock, shares outstanding (in shares) 58,500,000 58,500,000 55,800,000 55,800,000
Equity Distribution Program        
Class of Stock        
Stock program, maximum amount authorized $ 500,000,000      
Proceeds from issuance of common stock $ 0 $ 440,300,000    
Maximum number of shares available for sale under agreement (in shares)   2,900,000    
Common stock price (in dollars per share)   $ 167.25    
Number of common shares issued (in shares)   2,700,000    
Number of shares net settled for cash (in shares)   200,000    
Average share price of shares net settled for cash (in dollars per share)   $ 144.59    
Common stock, shares outstanding (in shares)   0