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Common Stock (Details Textual) - USD ($)
$ in Thousands
 1 Months Ended 3 Months Ended
Mar. 31, 2018
Jun. 30, 2017
Apr. 30, 2017
Mar. 31, 2018
Mar. 31, 2017
Dec. 31, 2017
Class of Stock [Line Items]            
Proceeds From Issuance Of Common Stock   $ 108,000   $ (93) $ 150  
Common Stock, Shares Authorized 45,000,000     45,000,000   45,000,000
Common Stock [Member]            
Class of Stock [Line Items]            
Proceeds From Issuance Of Common Stock       $ 162,900    
Stock Issued During Period, Shares, New Issues   2,415,000   0    
Common Stock [Member] | Forward Sale Agreement [Member]            
Class of Stock [Line Items]            
Forward Sale Agreement Expires Date Mar. 01, 2019          
Common Stock, Shares Authorized 3,500,000     3,500,000    
ATM Program [Member]            
Class of Stock [Line Items]            
Gross Amount Remaining To Be Issued       $ 83,500    
ATM Program [Member] | Common Stock [Member]            
Class of Stock [Line Items]            
Value Of Equity Instruments To Be Issued     $ 200,000