Debt - Summary of Debt Repayments (Details) - USD ($) $ in Millions	3 Months Ended				12 Months Ended
	Dec. 31, 2021	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2021
Debt Instrument [Line Items]
Debt Extinguished					$ 3,127
Redemption premiums paid		$ 28
Non-cash gain (loss) on extinguishment of debt		5
Arkansas Teacher Retirement System Notes Payable
Debt Instrument [Line Items]
Debt Extinguished	$ 106
Environmental Revenue Bonds
Debt Instrument [Line Items]
Debt Extinguished	70			$ 89
Finance leases and all other obligations
Debt Instrument [Line Items]
Debt Extinguished	46
6.250% Senior Notes due 2026
Debt Instrument [Line Items]
Debt Extinguished	230	370	$ 18	32
Redemption premiums paid	7
Non-cash gain (loss) on extinguishment of debt	$ (2)
6.250% Senior Notes due 2026 | Senior Notes
Debt Instrument [Line Items]
Stated interest rate	6.25%				6.25%
6.875% Senior Notes due 2025
Debt Instrument [Line Items]
Debt Extinguished		718	$ 14	18
6.875% Senior Notes due 2025 | Senior Notes
Debt Instrument [Line Items]
Stated interest rate	6.875%				6.875%
6.625% 2029 Senior Secured Notes
Debt Instrument [Line Items]
Debt Extinguished		$ 180
6.625% 2029 Senior Secured Notes | Senior Notes
Debt Instrument [Line Items]
Stated interest rate	6.625%				6.625%
12.000% 2025 Senior Secured Notes
Debt Instrument [Line Items]
Debt Extinguished				1,056
Redemption premiums paid				181
Non-cash gain (loss) on extinguishment of debt				(71)
12.000% 2025 Senior Secured Notes | Senior Notes
Debt Instrument [Line Items]
Stated interest rate	12.00%				12.00%
Export-Import Credit Agreement
Debt Instrument [Line Items]
Debt Extinguished				180
Non-cash gain (loss) on extinguishment of debt				$ (3)