Income Taxes (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	9 Months Ended	12 Months Ended
	Dec. 31, 2017	Sep. 30, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Income Tax Disclosure [Abstract]
Federal statutory rate		21.00%	24.50%	35.00%	35.00%
Unrecognized tax benefits that would impact effective tax rate		$ 117	$ 117
Unrecognized tax benefits, income tax penalties and interest expense			2	$ (1)	$ 2
Unrecognized tax benefits, income tax penalties and interest accrued		23	23	16
Income taxes paid			680	1,420	950
Net operating losses and tax credits		$ (396)	(396)	(444)
Net income tax expense (benefit), tax reform	$ (43)		$ (189)	$ 0	$ 0
Net income tax expense (benefit), tax reform, per diluted share	$ (0.07)		$ (0.30)
Income tax expense (benefit), continuing operations, adjustment of deferred tax (asset) liability			$ (94)
Income tax expense (benefit) due to repatriation of foreign earnings and withholding taxes			35
Income tax benefit for repatriation of foreign earnings, prior periods			$ 130
Amount of net operating losses and tax credits expiring over varying periods within the next 20 years			More than half