Accounting Changes - Revenue Recognition (Details) - USD ($) $ in Millions	Sep. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
Accounting Changes
Deferred costs	$ 4,808
Deferred tax assets	4,436		$ 4,862
Retained earnings	158,612		$ 153,126
Accounting Standards Update 2014-09, Revenue from Contracts with Customers | Adjustments to convert to prior GAAP
Accounting Changes
Net contract assets		$ 557
Deferred costs		737
Deferred income		29
Deferred tax assets	179	(184)
Retained earnings	$ (507)	$ 524