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Fair Value Measurements (Schedule Of Carrying Values And Estimated Fair Values Of Debt Instruments) (Details) - USD ($)
$ in Millions
Sep. 30, 2015
Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Carrying Value $ 5,903.3 $ 5,920.4
Estimated Fair Value $ 4,815.1 $ 5,973.4
Three Point Two Five Percent Senior Notes Member    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt Instrument, Interest Rate, Stated Percentage 3.25% 3.25%
Carrying Value $ 0.0 $ 998.0
Estimated Fair Value $ 0.0 $ 1,018.3
Four Point Six Five Percent Marine Administration Bonds Including Current Maturities Member    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt Instrument, Interest Rate, Stated Percentage 4.65% 4.65%
Carrying Value $ 0.0 $ 27.0
Estimated Fair Value $ 0.0 $ 29.7
Four Point Three Three Percent Marine Administration Bonds Including Current Maturities Member    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt Instrument, Interest Rate, Stated Percentage 4.33% 4.33%
Carrying Value $ 0.0 $ 46.6
Estimated Fair Value $ 0.0 $ 46.8
Seven Point Two Zero Percent Debentures Member    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt Instrument, Interest Rate, Stated Percentage 7.20% 7.20%
Carrying Value $ 149.2 $ 149.2
Estimated Fair Value $ 136.7 $ 171.4
Seven Point Eight Seven Five Percent Senior Notes Member    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt Instrument, Interest Rate, Stated Percentage 7.875% 7.875%
Carrying Value $ 380.2 $ 381.2
Estimated Fair Value $ 253.0 $ 363.8
Eight Point Five Zero Percent Senior Notes Member    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt Instrument, Interest Rate, Stated Percentage 8.50% 8.50%
Carrying Value $ 570.8 $ 583.8
Estimated Fair Value $ 517.5 $ 611.8
Four Point Five Percent Senior Notes Member [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt Instrument, Interest Rate, Stated Percentage 4.50% 4.50%
Carrying Value $ 624.2 $ 624.2
Estimated Fair Value $ 487.5 $ 602.0
Five Point Two Percent Senior Notes [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt Instrument, Interest Rate, Stated Percentage 5.20% 5.20%
Carrying Value $ 697.5 $ 0.0
Estimated Fair Value $ 572.3 $ 0.0
Six Point Eight Seven Five Percent Senior Notes Due Two Thousand Twenty Member    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt Instrument, Interest Rate, Stated Percentage 6.875% 6.875%
Carrying Value $ 995.4 $ 1,008.2
Estimated Fair Value $ 886.5 $ 1,008.5
Five Point Seven Five Percent Senior Notes [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt Instrument, Interest Rate, Stated Percentage 5.75% 5.75%
Carrying Value $ 1,004.0 $ 622.3
Estimated Fair Value $ 706.9 $ 615.8
Four Point Seven Zero Percent Senior Notes Member    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt Instrument, Interest Rate, Stated Percentage 4.70% 4.70%
Carrying Value $ 1,482.0 $ 1,479.9
Estimated Fair Value $ 1,254.7 $ 1,505.3