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Note 3 - Financing Arrangements (Details Textual) - USD ($)
$ in Millions
Sep. 30, 2015
Dec. 31, 2015
Jun. 30, 2015
Dec. 31, 2014
Line of Credit [Member] | Senior Debt Obligations [Member]        
Line of Credit Facility, Maximum Borrowing Capacity $ 500.0   $ 500.0  
Debt Instrument, Term 5 years      
Long-term Line of Credit   $ 150.0   $ 183.0
Debt, Weighted Average Interest Rate   1.39%    
Senior Notes [Member] | Senior Notes, First Issuance [Member]        
Unsecured Debt   $ 250.0    
Debt Instrument, Interest Rate, Stated Percentage   2.40%    
Senior Notes [Member] | Senior Notes, Second Issuance [Member]        
Unsecured Debt   $ 250.0    
Debt Instrument, Interest Rate, Stated Percentage   3.85%    
Senior Notes [Member] | Senior Notes, Third Issuance [Member]        
Unsecured Debt   $ 350.0    
Debt Instrument, Interest Rate, Stated Percentage   3.30%    
Senior Debt Obligations [Member]        
Line of Credit Facility, Maximum Borrowing Capacity Increase       $ 250.0
Long-term Debt, Maturities, Repayments of Principal in Year Four   $ 252.3    
Long-term Debt, Maturities, Repayments of Principal in Year Five   150.0    
Long-term Debt, Maturities, Repayments of Principal after Year Five   $ 602.7