Income Taxes (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Income Tax Disclosure [Abstract]
Unrecognized tax benefits that would impact effective tax rate	$ 100
Unrecognized tax benefits, income tax penalties and interest expense	(1)	$ 2	$ (4)
Unrecognized tax benefits, income tax penalties and interest accrued	16	21
Net current deferred tax assets		400
Net noncurrent deferred tax liabilities		174
Income taxes paid	$ 1,420	950	$ 1,590
Amount of net operating losses and tax credits can be carried forward indefinitely	one-third
Net operating losses and tax credits	$ 444	$ 164
Amount of net operating losses and tax credits that expire in 10 years	one-third