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Senior Notes (Narrative) (Details) - USD ($)	3 Months Ended		12 Months Ended
Senior Notes (Narrative) (Details) - USD ($)	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