Debt - Schedule of Long-Term Debt (Detail) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Long-term debt	$ (6,312)	$ (6,807)
Less: amount due within one year	3	1
Long-term debt, excluding amount due within one year	6,309	6,806
5.72% Notes, due 2019
Debt Instrument [Line Items]
Long-term debt	$ 0	(500)
Long-term debt, interest rate	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)
Other
Debt Instrument [Line Items]
Long-term debt	$ (18)	$ (23)