RETIREMENT-RELATED BENEFITS - OCI and AOCI (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2022	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2021
Changes in AOCI for retirement-related benefits
Current period loss (gain)	$ (136)	$ 56	$ (175)	$ (72)
Curtailments and settlements	(4)	(10)	(10)	(3)
Amortization of net loss included in net periodic benefit cost	(16)	(5)	(40)	(51)
Current period prior service costs (credits)		3	(4)	$ (1)
Amortization for prior service costs (credits) included in net periodic benefit cost		(1)	(1)
Defined Benefit Pension Plans
Changes in AOCI for retirement-related benefits
Net loss (gain) at beginning of period		253	474
Current period loss (gain)		57	(172)
Curtailments and settlements		(10)	(10)
Amortization of net loss included in net periodic benefit cost		(4)	(39)
Net loss (gain) at end of period	474	296	253
Prior service costs (credits) at beginning of period		8	9
Current period prior service costs (credits)		(2)
Amortization for prior service costs (credits) included in net periodic benefit cost		(1)	(1)
Prior service costs (credits) at end of period	$ 9	4	8
Total amounts recognized in accumulated other comprehensive loss (income)		$ 301	$ 261

Amount, before tax, of accumulated other comprehensive (income) loss for defined benefit plan, that has not been recognized in net periodic benefit cost (credit).

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