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Common Stock (Details Textual) - USD ($)
$ / shares in Units, $ in Thousands
 1 Months Ended 3 Months Ended 9 Months Ended
May 06, 2016
May 31, 2016
Sep. 30, 2016
Sep. 30, 2016
Sep. 30, 2015
Mar. 27, 2015
Class of Stock [Line Items]            
Proceeds From Issuance Of Common Stock   $ 109,700   $ 124,099 $ 38,795  
ATM Program [Member]            
Class of Stock [Line Items]            
Proceeds From Issuance Of Common Stock     $ 12,200 14,900    
Gross Amount Remaining To Be Issued       $ 45,800    
Common Stock Average Price     $ 49.66 $ 47.21    
Common Stock [Member]            
Class of Stock [Line Items]            
Registration Payment Arrangement, Maximum Potential Consideration           $ 500,000
Stock Issued During Period, Shares, New Issues   2,875,000        
Common Stock [Member] | ATM Program [Member]            
Class of Stock [Line Items]            
Value Of Equity Instruments To Be Issued $ 100,000          
Stock Issued During Period, Shares, New Issues     246,565 315,607