Revenue Recognition - Reconciliation of Contract Balances (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2021	Dec. 31, 2020
Reconciliation of Contract Balances
Accounts receivable (net of allowances of $58 in 2021 and $91 in 2020)	$ 1,746	$ 1,746	$ 1,444
Accounts receivable, allowances	58	58	91
Contract assets	80	80	72
Deferred income (current)	875	875	854
Deferred income (noncurrent)	520	520	$ 543
Revenue recognized that was included in deferred income at the beginning of the period	$ 478	$ 726