Income Taxes (Details) - USD ($) $ in Millions		3 Months Ended		12 Months Ended
	Apr. 30, 2018	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2017
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
Other Tax Expense (Benefit)		$ 9
Total Impact of TCJA		852
One-time transaction tax post-1986 E&P		1,035
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Unrecognized Tax Benefits		1,240				$ 1,238
Unrecognized Tax Benefits that Would Impact Effective Tax Rate		1,157				1,150
Incremental tax liability asserted by IRS		$ 1,162
Reported tax rate		8.00%	4.90%
Net tax expense/benefits related to interest and penalties		$ 17	$ 13
Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued, Total		675				$ 655
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent						35.00%
Deferred tax asset TCJA remeasurement benefit						$ 99
One-time transaction tax post-1986 E&P - net payment		$ 454
Subsequent Event [Member]
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Litigation Settlement, Amount	$ 275
Potential Reduction In Unrecognized Tax Benefits Over Next Twelve Months As Result Of Concluding Certain Matters					$ 897
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent				21.00%