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Senior Notes (Narrative) (Details) - USD ($) | 3 Months Ended | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | ---
Dec. 31, 2013 | Dec. 31, 2011 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2015 | Dec. 31, 2014 | Sep. 25, 2020 | Aug. 11, 2020 | Mar. 31, 2018
Fair Value Measurement [Domain] |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Liabilities Subject to Compromise |  |  | $ 940,800,000 |  |  |  |  |  |  |  |
Debtor-in-Possession Financing, Amount Arranged |  |  |  |  |  |  |  |  | $ 500,000,000.0 | $ 500,000,000.0 |
Debtor-in-Possession Financing, Borrowings Outstanding |  |  | 0 |  |  |  |  |  |  |  |
Liabilities Subject to Compromise |  |  | $ 7,313,700,000 | $ 0 |  |  |  |  |  |  |
Debentures Due 2027 |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Redemption Price, Percentage |  |  | 100.00% |  |  |  |  |  |  |  |
Reported Value Measurement [Member] |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Long-term Debt |  |  | $ 7,096,300,000 | 6,048,300,000 |  |  |  |  |  |  |
Liabilities Subject to Compromise |  |  | $ 7,096,300,000 |  |  |  |  |  |  |  |
7.38%SeniorNotesDue2025 [Member] |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt instrument, interest rate, stated percentage |  |  | 7.375% |  |  |  |  |  |  |  |
5.40%SeniorNotesDue2042 [Member] |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt instrument, interest rate, stated percentage |  |  | 5.40% |  |  |  |  |  |  |  |
5.85%SeniorNotesDue2044 [Member] |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt instrument, interest rate, stated percentage |  |  | 5.85% |  |  |  |  |  |  |  |
3.00% Exchangeable senior notes due 2024 |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt instrument, interest rate, stated percentage |  |  | 3.00% |  |  |  |  |  |  |  |
4.875%SeniorNotesDue2022Member [Member] |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt instrument, interest rate, stated percentage |  |  | 4.875% |  |  |  |  |  |  |  |
4.75%SeniorNotesDue2024Member [Member] |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt instrument, interest rate, stated percentage |  |  | 4.75% |  |  |  |  |  |  |  |
7.75% Senior notes due 2026 |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt instrument, interest rate, stated percentage |  |  | 7.75% |  |  |  |  |  |  |  |
5.20% Senior notes due 2025 |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt instrument, interest rate, stated percentage |  |  | 5.20% |  |  |  |  |  |  |  |
5.75% Senior notes due 2044 |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt instrument, interest rate, stated percentage |  |  | 5.75% |  |  |  |  |  |  |  |
4.50% Senior notes due 2024 |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt instrument, interest rate, stated percentage |  |  | 4.50% |  |  |  |  |  |  |  |
4.70% Senior notes due 2021 |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt instrument, interest rate, stated percentage |  |  | 4.70% |  |  |  |  |  |  |  |
6.875% Senior notes due 2020 |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt instrument, interest rate, stated percentage |  |  | 6.875% |  |  |  |  |  |  |  |
7.875% Senior notes due 2040 |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt instrument, interest rate, stated percentage |  |  | 7.875% |  |  |  |  |  |  |  |
7.20% Senior notes due 2027 |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt instrument, interest rate, stated percentage |  |  | 7.20% |  |  |  |  |  |  |  |
8.00% Senior notes due 2024 |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt instrument, interest rate, stated percentage |  |  | 8.00% |  |  |  |  |  |  |  |
Convertible Debt [Member] | 3.00% Exchangeable senior notes due 2024 |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Face Amount |  |  | $ 849,500,000 | 849,500,000 |  | $ 849,500,000 |  |  |  |  |
Conversion rate per $1,000 principal amount of shares |  |  | 17.8336 |  |  |  |  |  |  |  |
Debt Instrument, Convertible, Conversion Price |  |  | $ 56.08 |  |  |  |  |  |  |  |
Interest Expense, Debt, Excluding Amortization |  |  | $ 16,100,000 |  | $ 25,500,000 |  |  |  |  |  |
Amortization of Debt Issuance Costs and Discounts |  |  | 21,400,000 | 32,500,000 | 31,000,000.0 |  |  |  |  |  |
Convertible Debt [Member] | 3.00% Exchangeable senior notes due 2024 | Reported Value Measurement [Member] |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Long-term Debt |  |  | 849,500,000 | 699,000,000.0 |  |  |  |  |  |  |
Convertible Debt [Member] | Three percent exchangeable senior notes due twenty twenty four [Member] | Reported Value Measurement [Member] |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Long-term Debt |  |  | 849,500,000 |  |  |  |  |  |  |  |
Senior Notes [Member] | Reported Value Measurement [Member] |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Liabilities Subject to Compromise |  |  | $ 6,500,000,000 |  |  |  |  |  |  |  |
Senior Notes [Member] | 7.38%SeniorNotesDue2025 [Member] |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt instrument, interest rate, stated percentage |  |  | 7.375% |  |  |  |  |  |  |  |
Debt Instrument, Face Amount |  |  | $ 500,000,000.0 |  |  |  |  |  |  |  |
Senior Notes [Member] | 7.38%SeniorNotesDue2025 [Member] | Reported Value Measurement [Member] |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Long-term Debt |  |  | 360,800,000 | 329,200,000 |  |  |  |  |  |  |
Senior Notes [Member] | 5.40%SeniorNotesDue2042 [Member] |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Face Amount |  |  | 400,000,000.0 |  |  |  |  |  |  |  |
Senior Notes [Member] | 5.40%SeniorNotesDue2042 [Member] | Reported Value Measurement [Member] |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Long-term Debt |  |  | $ 400,000,000.0 | 262,800,000 |  |  |  |  |  |  |
Senior Notes [Member] | 5.85%SeniorNotesDue2044 [Member] |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt instrument, interest rate, stated percentage |  |  | 5.85% |  |  |  |  |  |  |  |
Debt Instrument, Face Amount |  |  | $ 400,000,000.0 |  |  |  |  |  |  |  |
Senior Notes [Member] | 5.85%SeniorNotesDue2044 [Member] | Reported Value Measurement [Member] |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Long-term Debt |  |  | $ 400,000,000.0 | 268,800,000 |  |  |  |  |  |  |
Senior Notes [Member] | 7.875% Senior Notes Due 2040 [Member] |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt instrument, interest rate, stated percentage |  |  | 7.875% |  |  |  |  |  |  |  |
Debt Instrument, Face Amount |  |  | $ 201,400,000 |  |  |  |  |  |  |  |
Senior Notes [Member] | 4.875%SeniorNotesDue2022Member [Member] |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt instrument, interest rate, stated percentage |  |  | 4.875% |  |  |  |  |  |  |  |
Debt Instrument, Face Amount |  |  | $ 620,800,000 |  |  |  |  |  |  |  |
Senior Notes [Member] | 4.875%SeniorNotesDue2022Member [Member] | Reported Value Measurement [Member] |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Long-term Debt |  |  | $ 620,800,000 | 599,200,000 |  |  |  |  |  |  |
Senior Notes [Member] | 4.75%SeniorNotesDue2024Member [Member] |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt instrument, interest rate, stated percentage |  |  | 4.75% |  |  |  |  |  |  |  |
Debt Instrument, Face Amount |  |  | $ 398,100,000 |  |  |  |  |  |  |  |
Senior Notes [Member] | 4.75%SeniorNotesDue2024Member [Member] | Reported Value Measurement [Member] |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Long-term Debt |  |  | 318,600,000 | 276,500,000 |  |  |  |  |  |  |
Senior Notes [Member] | 7.75% Senior notes due 2026 |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt instrument, interest rate, stated percentage |  |  |  |  |  |  |  |  |  |  | 7.75%
Debt Issuance Costs, Gross |  |  |  |  |  |  |  |  |  |  | $ 16,500,000
Debt Instrument, Face Amount |  |  |  | 1,000,000,000.0 | $ 1,000,000,000.0 |  |  |  |  |  |
Senior Notes [Member] | 7.75% Senior notes due 2026 | Reported Value Measurement [Member] |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Long-term Debt |  |  | 1,000,000,000.0 | 987,100,000 |  |  |  |  |  |  |
Senior Notes [Member] | 5.20% Senior notes due 2025 |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt instrument, interest rate, stated percentage |  |  |  |  |  |  | 5.20% |  |  |  |
Debt Instrument, Face Amount |  |  |  |  |  |  | $ 700,000,000.0 |  |  |  |
Senior Note Maturity Year |  |  |  |  |  |  | 2025 |  |  |  |
Debt Instrument, Unamortized Discount (Premium), Net |  |  |  |  |  |  | $ 2,600,000 |  |  |  |
Senior Notes [Member] | 5.20% Senior notes due 2025 | Reported Value Measurement [Member] |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Long-term Debt |  |  | 333,700,000 | 331,700,000 |  |  |  |  |  |  |
Senior Notes [Member] | 5.75% Senior notes due 2044 |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt instrument, interest rate, stated percentage |  |  |  |  |  |  | 5.75% | 5.75% |  |  |
Debt Instrument, Face Amount |  |  |  |  |  |  | $ 400,000,000.0 | $ 625,000,000.0 |  |  |
Senior Note Maturity Year |  |  |  |  |  |  | 2044 | 2044 |  |  |
Debt Instrument, Unamortized Discount (Premium), Net |  |  |  |  |  |  | $ 18,700,000 | $ 2,800,000 |  |  |
Senior Notes [Member] | 5.75% Senior notes due 2044 | Reported Value Measurement [Member] |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Long-term Debt |  |  | 1,000,500,000 | 973,300,000 |  |  |  |  |  |  |
Senior Notes [Member] | 4.50% Senior notes due 2024 |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt instrument, interest rate, stated percentage |  |  |  |  |  |  |  | 4.50% |  |  |
Debt Instrument, Face Amount |  |  |  |  |  |  |  | $ 625,000,000.0 |  |  |
Senior Note Maturity Year |  |  |  |  |  |  |  | 2024 |  |  |
Debt Instrument, Unamortized Discount (Premium), Net |  |  |  |  |  |  |  | $ 900,000 |  |  |
Senior Notes [Member] | 4.50% Senior notes due 2024 | Reported Value Measurement [Member] |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Long-term Debt |  |  | 303,400,000 | 302,000,000.0 |  |  |  |  |  |  |
Senior Notes [Member] | 4.70% Senior notes due 2021 |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt instrument, interest rate, stated percentage |  | 4.70% |  |  |  |  |  |  |  |  |
Debt Instrument, Face Amount |  | $ 1,500,000,000 |  |  |  |  |  |  |  |  |
Senior Note Maturity Year |  | 2021 |  |  |  |  |  |  |  |  |
Debt Instrument, Unamortized Discount (Premium), Net |  | $ 29,600,000 |  |  |  |  |  |  |  |  |
Senior Notes [Member] | 4.70% Senior notes due 2021 | Reported Value Measurement [Member] |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Long-term Debt |  |  | 100,700,000 | 113,200,000 |  |  |  |  |  |  |
Senior Notes [Member] | 6.875% Senior notes due 2020 |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt instrument, interest rate, stated percentage | 6.875% |  |  |  |  |  |  |  |  |  |
Debt Instrument, Face Amount | $ 900,000,000.0 |  |  |  |  |  |  |  |  |  |
Senior Note Maturity Year | 2020 |  |  |  |  |  |  |  |  |  |
Senior Notes [Member] | 6.875% Senior notes due 2020 | Reported Value Measurement [Member] |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Long-term Debt |  |  | 122,900,000 | 124,800,000 |  |  |  |  |  |  |
Senior Notes [Member] | 8.50% Senior notes due 2019(1) |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt instrument, interest rate, stated percentage | 8.50% |  |  |  |  |  |  |  |  |  |
Debt Instrument, Face Amount | $ 500,000,000.0 |  |  |  |  |  |  |  |  |  |
Senior Note Maturity Year | 2019 |  |  |  |  |  |  |  |  |  |
Senior Notes [Member] | 7.875% Senior notes due 2040 |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Face Amount | $ 300,000,000.0 |  |  |  |  | $ 300,000,000.0 |  |  |  |  |
Senior Notes [Member] | 7.875% Senior notes due 2040 | Reported Value Measurement [Member] |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Long-term Debt |  |  | 300,000,000.0 | $ 373,300,000 |  |  |  |  |  |  |
Senior Notes [Member] | 7.20% Senior notes due 2027 |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt instrument, interest rate, stated percentage |  |  |  | 7.20% |  |  |  |  |  |  |
Debt Instrument, Face Amount |  |  |  | $ 150,000,000.0 |  |  |  |  |  |  |
Senior Notes [Member] | 8.00% Senior notes due 2024 |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt instrument, interest rate, stated percentage |  |  |  |  |  | 8.00% |  |  |  |  |
Debt Instrument, Face Amount |  |  |  |  |  | $ 332,000,000.0 |  |  |  |  |
Senior Note Maturity Year |  |  |  |  |  | 2024 |  |  |  |  |
Senior Notes [Member] | 8.00% Senior notes due 2024 | Reported Value Measurement [Member] |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Long-term Debt |  |  | $ 292,300,000 | $ 295,700,000 |  |  |  |  |  |  |

X
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