Accounting Changes - Tax Reform and Leases (Details) - USD ($) $ in Millions		6 Months Ended
	Jan. 01, 2018	Jun. 30, 2019		Jan. 01, 2019
Leases
Package of practical expedients		true
Operating right-of-use assets		$ 4,998	[1]	$ 4,800
Financing right-of-use assets				200
Operating lease liabilities		5,265		5,100
Financing lease liabilities		$ 71		$ 200
Stranded tax effects
Reclassification from AOCI to retained earnings for stranded tax effects of U.S. tax reform	$ (2,400)

[1] Reflects the adoption of the FASB guidance on leases. Refer to note 2, “Accounting Changes” and note 5, “Leases.