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Long-Term Debt (Details) - USD ($)
$ in Millions
Nov. 07, 2012
Dec. 31, 2018
Jun. 30, 2018
Jun. 07, 2018
Long-Term Debt (Additional Textual) [Abstract]        
Fixed rate at fair market value   $ 597.5 $ 996.4  
2018 Credit Agreement [Member]        
Long-Term Debt (Textual) [Abstract]        
Borrowing outstanding under 2018 Credit Agreement   $ 0.0    
Line of Credit Facility, Maximum Borrowing Capacity       $ 700.0
2.650% Senior Notes due 2019 [Member]        
Long-Term Debt (Additional Textual) [Abstract]        
Proceeds from Issuance of Senior Long-term Debt $ 400.0      
Debt Instrument, Interest Rate, Stated Percentage 2.65%