Retirement-Related Benefits - Net Periodic Benefit Cost of Retirement-Related Benefit Plans (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Cost/(Income) of Pension Plans
Cost of defined contribution expense	$ 39	$ 49	$ 129	$ 144
Defined Benefit Pension Plans
Cost/(Income) of Pension Plans
Service cost	20	26	65	79
Interest cost	$ 2	$ 2	$ 5	$ 7
Interest cost - income statement location	Other (income) and expense	Other (income) and expense	Other (income) and expense	Other (income) and expense
Expected return on plan assets	$ (7)	$ (6)	$ (18)	$ (18)
Expected return on plan assets - income statement location	Other (income) and expense	Other (income) and expense	Other (income) and expense	Other (income) and expense
Recognized actuarial losses (gains)	$ 12	$ 9	$ 32	$ 27
Recognized actuarial losses - income statement location	Other (income) and expense	Other (income) and expense	Other (income) and expense	Other (income) and expense
Multi-employer plans/other costs	$ 1	$ 4	$ 6	$ 10
Net periodic benefit cost	27	35	90	105
Non Pension Postretirement Benefit Plans
Cost/(Income) of Pension Plans
Service cost	$ 1	$ 1	2	2
Interest cost			$ 1	$ 1
Interest cost - income statement location	Other (income) and expense	Other (income) and expense	Other (income) and expense	Other (income) and expense
Expected return on plan assets			$ (1)	$ (1)
Expected return on plan assets - income statement location	Other (income) and expense	Other (income) and expense	Other (income) and expense	Other (income) and expense
Recognized actuarial losses - income statement location	Other (income) and expense	Other (income) and expense	Other (income) and expense	Other (income) and expense
Net periodic benefit cost	$ 1	$ 1	$ 2	$ 2