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Fair Value Measurement (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended
Dec. 31, 2011
Mar. 31, 2012
Senior Notes [Member]
Mar. 31, 2012
Debentures With Put Feature [Member]
Mar. 31, 2012
Fair Value, Measurements, Recurring [Member]
Dec. 31, 2011
Fair Value, Measurements, Recurring [Member]
Mar. 31, 2012
Fair Value, Measurements, Recurring [Member]
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Dec. 31, 2011
Fair Value, Measurements, Recurring [Member]
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Mar. 31, 2012
Fair Value, Measurements, Recurring [Member]
Significant Other Observable Inputs (Level 2) [Member]
Dec. 31, 2011
Fair Value, Measurements, Recurring [Member]
Significant Other Observable Inputs (Level 2) [Member]
Mar. 31, 2012
Fair Value, Measurements, Recurring [Member]
Significant Unobservable Inputs (Level 3) [Member]
Dec. 31, 2011
Fair Value, Measurements, Recurring [Member]
Significant Unobservable Inputs (Level 3) [Member]
Mar. 31, 2012
Fair Value, Measurements, Nonrecurring [Member]
Dec. 31, 2011
Fair Value, Measurements, Nonrecurring [Member]
Mar. 31, 2012
Fair Value, Measurements, Nonrecurring [Member]
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Dec. 31, 2011
Fair Value, Measurements, Nonrecurring [Member]
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Mar. 31, 2012
Fair Value, Measurements, Nonrecurring [Member]
Significant Other Observable Inputs (Level 2) [Member]
Dec. 31, 2011
Fair Value, Measurements, Nonrecurring [Member]
Significant Other Observable Inputs (Level 2) [Member]
Mar. 31, 2012
Fair Value, Measurements, Nonrecurring [Member]
Significant Unobservable Inputs (Level 3) [Member]
Dec. 31, 2011
Fair Value, Measurements, Nonrecurring [Member]
Significant Unobservable Inputs (Level 3) [Member]
Assets, fair value measurements [Abstract]                                      
Marketable securities       $ 12.9 $ 10.4 $ 12.9 $ 10.4 $ 0 $ 0 $ 0 $ 0                
Derivative instruments       9.6 9.3 0 0 9.6 9.3 0 0                
Cash and Cash Equivalents, Fair Value Disclosure                       1,089.5 1,160.7 1,089.5 1,160.7 0 0 0 0
Total assets, fair value measurements       22.5 19.7 12.9 10.4 9.6 9.3 0 0 1,089.5 1,160.7 1,089.5 1,160.7 0 0 0 0
Liabilities, fair value measurements [Abstract]                                      
Derivative instruments       11.3 22.2 0 0 11.3 22.2 0 0                
Long-term debt                       4,544.2 4,359.2 0 0 4,544.2 4,359.2 0 0
Total liability, fair value measurements       11.3 22.2 0 0 11.3 22.2 0 0 4,544.2 4,359.2 0 0 4,544.2 4,359.2 0 0
Benefit trust assets 169.5                                    
Benefit trust liabilities $ 178.3                                    
Debt instrument, maturity date range, start     2027                                
Debt instrument, maturity date range, end   2025 2028