Debt - Schedule of Long-Term Debt (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Long-term debt	$ 6,304	$ 6,301
Less: amount due within one year	405	405
Long-term debt, excluding amount due within one year	5,899	5,896
1.63% Convertible Notes, due 2019
Debt Instrument [Line Items]
Long-term debt	$ 403	403
Long-term debt, interest rate	1.63%
6.150% Notes, due 2020
Debt Instrument [Line Items]
Long-term debt	$ 1,000	1,000
Long-term debt, interest rate	6.15%
5.40% Notes due 2021
Debt Instrument [Line Items]
Long-term debt	$ 1,250	1,250
Long-term debt, interest rate	5.40%
5.87% Notes, due 2022
Debt Instrument [Line Items]
Long-term debt	$ 627	627
Long-term debt, interest rate	5.87%
5.125% Notes, due 2024
Debt Instrument [Line Items]
Long-term debt	$ 1,250	1,250
Long-term debt, interest rate	5.125%
5.90% Notes, due 2027
Debt Instrument [Line Items]
Long-term debt	$ 625	625
Long-term debt, interest rate	5.90%
6.75% Bonds, due 2028
Debt Instrument [Line Items]
Long-term debt	$ 300	300
Long-term debt, interest rate	6.75%
5.95% Notes, due 2037
Debt Instrument [Line Items]
Long-term debt	$ 625	625
Long-term debt, interest rate	5.95%
Iowa Finance Authority Loan, due 2042
Debt Instrument [Line Items]
Long-term debt	$ 250	250
Other
Debt Instrument [Line Items]
Other	$ (26)	$ (29)

Contractual interest rate for funds borrowed, under the debt agreement.

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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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Amount after unamortized (discount) premium and debt issuance costs of long-term debt classified as noncurrent and excluding amounts to be repaid within one year or the normal operating cycle, if longer. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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