Investments in Affiliates (Schedule of Summarized Balance Sheet Information for Ibn Sina (National Methanol)) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019
Schedule of summarized balance sheet information for Ibn Sina (National Methanol) [Line Items]
Assets, Current	$ 3,763		$ 2,765
Current liabilities	$ 1,973		$ 1,754
National Methanol Company (Ibn Sina) [Member] | Equity Method Investment, Nonconsolidated Investee or Group of Investees
Schedule of summarized balance sheet information for Ibn Sina (National Methanol) [Line Items]
Assets, Current		$ 244		$ 253
Assets, Noncurrent		817		871
Current liabilities		201		148
Liabilities, Noncurrent		$ 372		$ 433