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Debt and Credit Agreements (Details) - USD ($)
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Debt Instrument [Line Items]        
Gain (Loss) on Extinguishment of Debt $ (1,034,000) $ 0 $ (1,034,000) $ 0
Line of Credit [Member]        
Debt Instrument [Line Items]        
Maximum borrowing capacity $ 500,000,000   $ 500,000,000  
Line of Credit [Member] | LIBOR        
Debt Instrument [Line Items]        
Variable rate basis spread     2.25%