Related Party Transactions - Allocation of Corporate Expenses (Details) - IBM - Management Costs And Corporate Support Services - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Related Party Transaction [Line Items]
Allocation of corporate expenses	$ 310	$ 284	$ 977	$ 915
Selling, general and administrative expense
Related Party Transaction [Line Items]
Allocation of corporate expenses	292	266	922	860
Research, development and engineering expense
Related Party Transaction [Line Items]
Allocation of corporate expenses	(1)		(1)	3
Other (income) and expense.
Related Party Transaction [Line Items]
Allocation of corporate expenses	2	2	10	5
Interest expense
Related Party Transaction [Line Items]
Allocation of corporate expenses	$ 17	$ 16	$ 46	$ 47