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Debt - Public Debt (Details) - USD ($) |  |  |  |  | 3 Months Ended | 9 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Jan. 15, 2021 | May 21, 2020 | Apr. 06, 2020 | Oct. 15, 2019 | Mar. 31, 2021 | Jun. 30, 2020 | Sep. 30, 2020 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Apr. 24, 2020 | Mar. 31, 2018
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Early termination premium |  |  |  |  |  | $ 35,000,000 |  |  |  |  |  |
Interest expense |  |  |  |  |  |  | $ 17,000,000 |  |  | $ 19,000,000 |  |
Debt prepayment cost |  |  |  |  |  |  |  | $ 59,000,000 | $ 0 | $ 17,000,000 |  |
5.400% Notes, due 2021 |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Early redemption of debt |  | $ 589,000,000 | $ 300,000,000 |  |  |  |  |  |  |  |  |
Long- term debt, interest rate (percentage) |  | 5.40% | 5.40% |  |  |  |  | 5.40% |  |  |  |
Interest expense |  |  |  |  |  | 24,000,000 |  |  |  |  |  |
Debt prepayment cost |  |  | $ 315,000,000 |  |  |  |  |  |  |  |  |
5.400% Notes, due 2021 | Subsequent Event |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Early redemption of debt | $ 361,000,000 |  |  |  |  |  |  |  |  |  |  |
Long- term debt, interest rate (percentage) | 5.40% |  |  |  |  |  |  |  |  |  |  |
Interest expense | $ 19,000,000 |  |  |  |  |  |  |  |  |  |  |
5.400% Notes, due 2021 | Scenario, forecast |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Early termination premium |  |  |  |  | $ 5,000,000 |  |  |  |  |  |  |
5.870% Notes, due 2022 |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Early redemption of debt |  | $ 151,000,000 |  |  |  |  |  |  |  |  |  |
Long- term debt, interest rate (percentage) |  | 5.87% |  |  |  |  |  | 5.87% |  |  |  |
Interest expense |  |  |  |  |  |  | $ 4,000,000 |  |  |  |  |
6.875% Notes, due 2025 |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Long- term debt, interest rate (percentage) |  |  |  |  |  |  |  | 6.875% |  |  | 6.875% |
Aggregate principal amount |  |  |  |  |  |  |  |  |  |  | $ 1,200,000,000 |
Deferred financing costs |  |  |  |  |  | $ 14,000,000 |  |  |  |  |  |
6.150% Notes, due 2020 |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Early redemption of debt |  |  | $ 1,000,000,000 |  |  |  |  |  |  |  |  |
Long- term debt, interest rate (percentage) |  |  | 6.15% |  |  |  |  | 6.15% |  |  |  |
Debt prepayment cost |  |  | $ 1,020,000,000 |  |  |  |  |  |  |  |  |
1.63% convertible notes |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Early redemption of debt |  |  |  | $ 403,000,000 |  |  |  |  |  |  |  |
Long- term debt, interest rate (percentage) |  |  |  | 1.63% |  |  |  |  |  |  |  |
5.72% Notes due 2019 |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Long- term debt, interest rate (percentage) |  |  |  |  |  |  |  |  |  |  |  | 5.72%
Aggregate principal amount |  |  |  |  |  |  |  |  |  |  |  | $ 500,000,000
Debt instrument redemption amount |  |  |  |  |  |  |  |  |  |  |  | $ 518,000,000

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