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Common Stock (Details Textual) - USD ($)
$ in Thousands
 1 Months Ended 3 Months Ended 6 Months Ended
May 31, 2018
Mar. 31, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Class of Stock [Line Items]              
Proceeds From Issuance Of Common Stock         $ (432) $ 107,987  
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          
ATM Program [Member]              
Class of Stock [Line Items]              
Gross Amount Remaining To Be Issued         $ 250,000    
ATM Program [Member] | Common Stock [Member]              
Class of Stock [Line Items]              
Value Of Equity Instruments To Be Issued $ 250,000