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Debt - Schedule of Long-Term Debt (Details) - USD ($) $ in Millions	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Long-term debt	$ 6,315		$ 6,807
Other	(15)		(23)
Less: amount due within one year	3		1
Long-term debt, excluding amount due within one year	6,312		6,806
5.72% Notes, due 2019
Debt Instrument [Line Items]
Long-term debt	$ 0		500
Long-term debt, interest rate	5.72%	5.72%
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