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Common Stock (Details Textual) - USD ($)
$ / shares in Units, $ in Thousands
 1 Months Ended 12 Months Ended
May 06, 2015
Oct. 31, 2016
May 31, 2016
Dec. 31, 2015
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Mar. 31, 2015
Class of Stock [Line Items]                
Proceeds From Issuance Of Common Stock   $ 95,000 $ 109,600 $ 53,000 $ 228,011 $ 92,260 $ 73,302  
ATM Program [Member]                
Class of Stock [Line Items]                
Proceeds From Issuance Of Common Stock         23,800      
Gross Amount Remaining To Be Issued         $ 36,200      
Common Stock Average Price         $ 47.74      
Common Stock [Member]                
Class of Stock [Line Items]                
Registration Payment Arrangement, Maximum Potential Consideration               $ 500,000
Stock Issued During Period, Shares, New Issues   2,087,250 2,875,000 1,725,000        
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         499,209