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Common Shares (Narrative) (Details) - USD ($)
$ in Thousands
 1 Months Ended 6 Months Ended
May 31, 2017
Nov. 30, 2014
Jun. 30, 2018
Aug. 03, 2018
Dec. 31, 2017
May 05, 2017
Common shares repurchased     $ 253      
Treasury Stock, Shares, Acquired     3,222      
Number of common and preferred stock authorized to issue     185,000,000      
Common shares, authorized     175,000,000   175,000,000  
Preferred shares, authorized     10,000,000      
Common Stock, Shares, Outstanding     92,800,000      
Preferred Stock, Shares Outstanding     0      
Unsecured Credit Facility [Member]            
Line of Credit Facility, Current Borrowing Capacity     $ 600,000      
2014 ATM program [Member]            
Maximum aggregate offering price of remaining common shares available for sale           $ 315,300
Maximum aggregate offering price of common shares   $ 331,300        
2017 ATM program [Member]            
Maximum aggregate offering price of common shares $ 315,300          
2017 ATM program [Member] | Subsequent Event [Member]            
Maximum aggregate offering price of remaining common shares available for sale       $ 312,800    
April 2007 Repurchase Plan [Member]            
Treasury stock allowed for repurchase     $ 500,000      
April 2007 Repurchase Plan [Member] | Subsequent Event [Member]            
Share Repurchase Program, Remaining Authorized Repurchase Amount       $ 269,500