Debt - Schedule of Long-Term Debt (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Long-term debt	$ 5,909	$ 6,301
Less: amount due within one year	1,003	405
Long-term Debt, Excluding Current Maturities	4,906	5,896
1.63% Convertible Notes, due 2019
Debt Instrument [Line Items]
Long-term debt	0	403
6.150% Notes, due 2020
Debt Instrument [Line Items]
Long-term debt	1,000	1,000
5.40% Notes, due 2021
Debt Instrument [Line Items]
Long-term debt	1,250	1,250
5.87% Notes, due 2022
Debt Instrument [Line Items]
Long-term debt	627	627
5.125% Notes, due 2024
Debt Instrument [Line Items]
Long-term debt	1,250	1,250
5.90% Notes, due 2027
Debt Instrument [Line Items]
Long-term debt	625	625
6.75% Bonds, due 2028
Debt Instrument [Line Items]
Long-term debt	300	300
5.95% Notes due 2037
Debt Instrument [Line Items]
Long-term debt	625	625
Iowa Finance Authority Loan, due 2042 (4.75%)
Debt Instrument [Line Items]
Long-term debt	250	250
Other
Debt Instrument [Line Items]
Long-term debt	$ 18	$ 29

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt, classified as current. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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