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Fair Value Measurements (Schedule Of Carrying Values And Estimated Fair Values Of Debt Instruments) (Details) - USD ($)
$ in Millions
Sep. 30, 2017
Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Carrying Value $ 4,747.7 $ 5,274.5
Estimated Fair Value $ 4,251.0 $ 5,001.7
8.50% Senior notes due 2019    
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 $ 253.7 $ 480.2
Estimated Fair Value $ 252.3 $ 485.0
6.875% Senior notes due 2020    
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 $ 480.3 $ 735.9
Estimated Fair Value $ 465.2 $ 727.5
4.70% Senior notes due 2021    
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 $ 266.9 $ 674.4
Estimated Fair Value $ 264.6 $ 658.9
3.00% Exchangeable senior notes due 2024    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt instrument interest rate stated percentage 3.00% 3.00%
Carrying Value $ 628.2 $ 604.3
Estimated Fair Value $ 726.8 $ 874.7
4.50% Senior notes due 2024    
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 $ 619.1 $ 618.6
Estimated Fair Value $ 520.2 536.0
8.00% Senior notes due 2024    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt instrument interest rate stated percentage 8.00%  
Carrying Value $ 338.2 0.0
Estimated Fair Value $ 330.2 $ 0.0
5.20% Senior notes due 2025    
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 $ 663.4 $ 662.8
Estimated Fair Value $ 564.0 $ 582.3
7.20% Debentures due 2027    
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 $ 139.2 $ 138.7
7.875% Senior notes due 2040    
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 $ 377.1 $ 378.3
Estimated Fair Value $ 256.6 $ 270.6
5.75% Senior notes due 2044    
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 $ 971.6 $ 970.8
Estimated Fair Value 731.9 728.0
Convertible Debt [Member] | 3.00% Exchangeable senior notes due 2024    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
3.00% exchangeable senior notes, excluding discount and net of debt issuance costs $ 833.5 $ 830.1