Revenue Recognition - Deferred Costs (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2021	Dec. 31, 2020
Capitalized Contract Cost, Net [Abstract]
Capitalized costs to obtain a contract	$ 285	$ 285	$ 269
Deferred costs to fulfill a contract
Deferred transition and setup costs	1,195	1,195	1,369
Other deferred fulfillment costs	1,016	1,016	1,006
Total deferred costs	2,496	2,496	2,646
Deferred costs current	1,142	1,142	1,205
Deferred costs noncurrent	1,354	1,354	1,441
Total deferred costs amortized	447	1,427
Amortization of prepaid software	215	685
Amortization of transition and setup costs	97	291
Amortization of other deferred fulfilment costs	135	$ 451
Minimum
Capitalized Contract Cost [Line Items]
Term of contract with customer		3 years
Maximum
Capitalized Contract Cost [Line Items]
Term of contract with customer		6 years
IBM
Deferred costs to fulfill a contract
Other deferred fulfillment costs	78	$ 78	94
Deferred costs current	66	66	76
Deferred costs noncurrent	$ 12	$ 12	$ 18