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Debt - Schedule of Long-Term Debt (Details) - USD ($) $ in Millions	Jun. 30, 2020	May 21, 2020	Apr. 24, 2020	Apr. 06, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Long-term debt	$ 5,086				$ 5,940
Less: amount due within one year	391				1,034
Total long-term debt	$ 4,695				4,906
6.150% Notes, due 2020
Debt Instrument [Line Items]
Long-term debt, interest rate (percentage)	6.15%			6.15%
Long-term debt	$ 0				1,000
5.400% Notes due 2021
Debt Instrument [Line Items]
Long-term debt, interest rate (percentage)	5.40%	5.40%		5.40%
Long-term debt	$ 361				1,250
5.870% Notes, due 2022
Debt Instrument [Line Items]
Long-term debt, interest rate (percentage)	5.87%	5.87%
Long-term debt	$ 476				627
5.125% Notes, due 2024
Debt Instrument [Line Items]
Long-term debt, interest rate (percentage)	5.125%
Long-term debt	$ 1,250				1,250
6.875% Notes, due 2025
Debt Instrument [Line Items]
Long-term debt, interest rate (percentage)	6.875%		6.875%
Long-term debt	$ 1,200				0
5.900% Notes, due 2027
Debt Instrument [Line Items]
Long-term debt, interest rate (percentage)	5.90%
Long-term debt	$ 625				625
6.750% Bonds, due 2028
Debt Instrument [Line Items]
Long-term debt, interest rate (percentage)	6.75%
Long-term debt	$ 300				300
5.950% Notes, due 2037
Debt Instrument [Line Items]
Long-term debt, interest rate (percentage)	5.95%
Long-term debt	$ 625				625
4.750% Iowa Finance Authority Loan, due 2042
Debt Instrument [Line Items]
Long-term debt, interest rate (percentage)	4.75%
Long-term debt	$ 250				250
Other
Debt Instrument [Line Items]
Other	$ (1)				$ 13