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Long-Term Debt (Details) - USD ($)
$ in Millions
9 Months Ended
Sep. 30, 2015
Sep. 30, 2014
Debt Instrument Issuance [Abstract]    
Net proceeds from Issuance of Senior Long-Term Debt $ 990.0  
Revolving Credit Agreement [Member]    
Line of Credit Facility [Line Items]    
Borrowings Outstanding - End of Period $ 0.0  
Line of Credit Facility, Expiration Date Oct. 11, 2016  
Letter of Credit Facility Previous Maximum Capacity $ 2,000.0  
Uncommitted Credit Facility [Member]    
Line of Credit Facility [Line Items]    
Borrowings Outstanding - End of Period 0.0 $ 0.0
Average Outstanding Amount 0.0 $ 0.1
Weighted average interest rate (in hundredths)   0.70%
Commercial Paper [Member]    
Line of Credit Facility [Line Items]    
Borrowings Outstanding - End of Period 30.0 $ 0.0
Average Outstanding Amount $ 7.0 $ 16.0
Weighted average interest rate (in hundredths) 0.45% 0.25%
New Facility [Member]    
Line of Credit Facility [Line Items]    
Letters of Credit Outstanding $ 0.0  
Borrowings Outstanding - End of Period $ 0.0  
Line of Credit Facility, Expiration Date Jul. 21, 2020  
Letter of Credit Facility Current Maximum Capacity $ 2,000.0  
Eurodollar rate at period end (in hundredths) 1.09%  
Base rate at period end (in hundredths) 3.25%  
Maximum total debt to capitalization ratio allowed under financial convenant 65.00%  
3.90% Senior Notes due 2035 [Member]    
Debt Instrument Issuance [Abstract]    
Debt Instrument Issuance Face Amount $ 500.0  
Debt Instrument Issuance Interest Rate 3.90%  
3.15 % Senior Notes due 2025 [Member]    
Debt Instrument Issuance [Abstract]    
Debt Instrument Issuance Face Amount $ 500.0  
Debt Instrument Issuance Interest Rate 3.15%  
2.500% Senior Notes Due 2016 [Member]    
Debt Instrument Issuance [Abstract]    
Current debt outstanding reclassified to Long-Term Debt on Consolidated Balance Sheet $ 400.0  
Debt Instrument Issuance Interest Rate 2.50%  
2.95% Senior Notes Due 2015 [Member]    
Debt Instrument Issuance [Abstract]    
Debt Instrument Issuance Interest Rate 2.95%  
Repayment of Debt $ 500.0  
Debt Instrument, Repayment Date Jun. 01, 2015