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Common Stock (Details) - USD ($)
$ / shares in Units, $ in Thousands
 1 Months Ended 12 Months Ended
Sep. 30, 2018
Jun. 30, 2017
Apr. 30, 2017
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Mar. 31, 2018
Class of Stock [Line Items]              
Proceeds From Issuance Of Common Stock       $ 339,744 $ 222,695 $ 228,011  
Common Stock, Shares Authorized       45,000,000 45,000,000    
Common Stock [Member]              
Class of Stock [Line Items]              
Stock Issued During Period, Shares, New Issues   2,415,000   6,507,263 4,786,604 5,461,459  
Proceeds From Issuance Of Common Stock   $ 108,000          
Common Stock [Member] | Forward Sale Agreement [Member]              
Class of Stock [Line Items]              
Stock Issued During Period, Shares, New Issues 3,450,000            
Proceeds From Issuance Of Common Stock $ 160,200            
Common Stock, Shares Authorized             3,450,000
Common Stock [Member] | Forward Contracts Two [Member]              
Class of Stock [Line Items]              
Common Stock, Shares Authorized 3,500,000            
ATM Program [Member]              
Class of Stock [Line Items]              
Proceeds From Issuance Of Common Stock       $ 181,200      
Gross Amount Remaining To Be Issued       $ 68,800      
Common stock Average Price       $ 59.28      
Over-Allotment Option [Member] | Common Stock [Member] | Forward Sale Agreement [Member]              
Class of Stock [Line Items]              
Common Stock, Shares Authorized             450,000
Common Stock [Member] | ATM Program [Member]              
Class of Stock [Line Items]              
Stock Issued During Period, Shares, New Issues       3,057,263      
Value Of Equity Instruments To Be Issued     $ 250,000