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Common Stock Common Stock (Equity Issuances) (Details) - USD ($)
$ in Thousands
 12 Months Ended 24 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2017
Class of Stock [Line Items]        
Common Stock, Shares Authorized 200,000,000 200,000,000   200,000,000
Common Stock [Member]        
Class of Stock [Line Items]        
Shares issued through sales agency agreements 1,070,000 1,645,000 0  
Issuance of common stock through sales agency agreements, net of issuance costs $ 54,721 $ 65,267 $ 0 $ 120,000
Sales Agency Agreement [Member]        
Class of Stock [Line Items]        
Common Stock, Shares Authorized 3,800,000     3,800,000
Shares issued through sales agency agreements       2,700,000
Common Stock Shares Authorized Under Sales Agency Agreements Remaining Shares Authorized To Sell 1,100,000     1,100,000