RETIREMENT-RELATED BENEFITS - Net Periodic Benefit Cost of Retirement-Related Benefit Plans (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Defined Benefit Pension Plans
Cost/(Income) of Pension Plans
Service cost	$ 14.0	$ 23.0
Interest cost	$ 6.0	$ 1.0
Interest cost - income statement location	Other expenses	Other expenses
Expected return on plan assets	$ (10.0)	$ (5.0)
Expected return on plan assets - income statement location	Other expenses	Other expenses
Amortization of prior service costs/(credits) - income statement location	Other expenses	Other expenses
Recognized actuarial losses (gains)	$ 16.0	$ 11.0
Recognized actuarial losses - income statement location	Other expenses	Other expenses
Curtailments and settlements	$ 4.0
Curtailments and settlements - income statement location	Other expenses	Other expenses
Multi-employer plans/other costs	$ (1.0)	$ 3.0
Net periodic benefit cost	$ 28.0	32.0
Non Pension Postretirement Benefit Plans
Cost/(Income) of Pension Plans
Interest cost		$ 1.0
Interest cost - income statement location	Other expenses	Other expenses
Amortization of prior service costs/(credits) - income statement location	Other expenses	Other expenses
Recognized actuarial losses - income statement location	Other expenses	Other expenses
Curtailments and settlements - income statement location	Other expenses	Other expenses
Net periodic benefit cost		$ 1.0