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Accounting Changes - Revenue Recognition (Details) - USD ($) $ in Millions		3 Months Ended	9 Months Ended	12 Months Ended
	Jan. 01, 2018	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2018	Sep. 30, 2019
Accounting Changes
Deferred tax assets		$ 5,216		$ 5,216	$ 4,994
Liability for taxes		3,046		3,046	$ 2,154
Accounting Standards Update 2014-09, Revenue from Contracts with Customers
Accounting Changes
Cumulative-effect net change in retained earnings			$ 524
Accounting Standards Update 2014-09, Revenue from Contracts with Customers \| Adjustments from Prior GAAP
Accounting Changes
Net contract assets	$ 557
Deferred costs	737
Deferred income	29
Deferred tax assets	(184)
Liability for taxes		(56)		(56)
Cumulative-effect net change in retained earnings	$ 524	$ 56		$ 580