Commitments - Extensions of Credit (Details) - USD ($) $ in Billions	Nov. 03, 2021	Sep. 30, 2021	Dec. 31, 2020
Commitments, guarantees:
Future financing commitments in support of IBM's managed infrastructure services unit (as a percentage)		35.00%
Commitments extinguished upon separation (as a percent)	100.00%
Extended lines of credit
Commitments, guarantees:
Unused amounts in lines of credit to third-party entities and commitments for future financing to clients		$ 1.7	$ 2.1
Financing for client purchase agreements
Commitments, guarantees:
Unused amounts in lines of credit to third-party entities and commitments for future financing to clients		$ 4.5	$ 5.2