Debt - Schedule of Long-Term Debt (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Long-term debt	$ 6,807	$ 8,048
Less: amount due within one year	1	4
Long-term debt, excluding amount due within one year	6,806	$ 8,044
6.50% Bonds, due 2018
Debt Instrument [Line Items]
Convertible notes, interest rate		6.50%
Long-term debt	0	$ 250
6.75% Notes, due 2018
Debt Instrument [Line Items]
Convertible notes, interest rate		6.75%
Long-term debt	$ 0	$ 750
5.72% Notes, due 2019
Debt Instrument [Line Items]
Convertible notes, interest rate	5.72%
Long-term debt	$ 500	750
1.63% Convertible Notes, Due 2019
Debt Instrument [Line Items]
Convertible notes, interest rate	1.63%
Long-term debt	$ 403	403
6.150% Notes, due 2020
Debt Instrument [Line Items]
Convertible notes, interest rate	6.15%
Long-term debt	$ 1,000	1,000
5.40% Notes, due 2021
Debt Instrument [Line Items]
Convertible notes, interest rate	5.40%
Long-term debt	$ 1,250	1,250
5.87% Notes, due 2022
Debt Instrument [Line Items]
Convertible notes, interest rate	5.87%
Long-term debt	$ 627	627
5.125% Notes, due 2024
Debt Instrument [Line Items]
Convertible notes, interest rate	5.125%
Long-term debt	$ 1,250	1,250
5.90% Notes, due 2027
Debt Instrument [Line Items]
Convertible notes, interest rate	5.90%
Long-term debt	$ 625	625
6.75% Bonds, due 2028
Debt Instrument [Line Items]
Convertible notes, interest rate	6.75%
Long-term debt	$ 300	300
5.95% Notes due 2037
Debt Instrument [Line Items]
Convertible notes, interest rate	5.95%
Long-term debt	$ 625	625
Iowa Finance Authority Loan, due 2042 (4.75%)
Debt Instrument [Line Items]
Convertible notes, interest rate	4.75%
Long-term debt	$ 250	250
Other
Debt Instrument [Line Items]
Other	$ (23)	$ (32)