Debt - Schedule of Long-Term Debt (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014	Sep. 30, 2014
Debt Instrument [Line Items]
Long-term debt	$ 9,065	$ 8,798	$ 1,250
Less: amount due within one year	21	29
Long-term debt, excluding amount due within one year	9,044	8,769
Long-term debt	9,065	8,798	$ 1,250
5.55% Notes, Due 2017 [Member]
Debt Instrument [Line Items]
Long-term debt	750	750
Long-term debt	750	750
6.50% Bonds, Due 2018 [Member]
Debt Instrument [Line Items]
Long-term debt	250	250
Long-term debt	250	250
6.75% Notes, Due 2018 [Member]
Debt Instrument [Line Items]
Long-term debt	750	750
Long-term debt	750	750
5.72% Notes, Due 2019 [Member]
Debt Instrument [Line Items]
Long-term debt	750	750
Long-term debt	750	750
1.63% Convertible Notes, Due 2019 [Member]
Debt Instrument [Line Items]
Long-term debt	403
Long-term debt	403
6.150% Notes, Due 2020 [Member]
Debt Instrument [Line Items]
Long-term debt	1,000	1,000
Long-term debt	1,000	1,000
5.40% Notes, Due 2021 [Member]
Debt Instrument [Line Items]
Long-term debt	1,250	1,250
Long-term debt	1,250	1,250
5.87% Notes, Due 2022 [Member]
Debt Instrument [Line Items]
Long-term debt	627	627
Long-term debt	627	627
5.125% Notes, Due 2024 [Member]
Debt Instrument [Line Items]
Long-term debt	1,250	1,250
Long-term debt	1,250	1,250
5.90% Notes, Due 2027 [Member]
Debt Instrument [Line Items]
Long-term debt	625	625
Long-term debt	625	625
6.75% Bonds, Due 2028 [Member]
Debt Instrument [Line Items]
Long-term debt	300	300
Long-term debt	300	300
5.95% Notes Due 2037 [Member]
Debt Instrument [Line Items]
Long-term debt	625	625
Long-term debt	625	625
BNDES Loans, Due 2015-2029 [Member]
Debt Instrument [Line Items]
Long-term debt	174	267
Long-term debt	174	267
Iowa Finance Authority Loan, Due 2042 (4.75%) [Member]
Debt Instrument [Line Items]
Long-term debt	250	250
Long-term debt	250	250
Other [Member]
Debt Instrument [Line Items]
Long-term debt	61	104
Long-term debt	$ 61	$ 104