Segment Information - Revenue (Net of Reimbursable Expenses) of the Reported Segments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Segment Reporting Information [Line Items]
Revenues	$ 9,483	$ 8,866	$ 8,998
Income from operations	1,365	1,178	2,202
HWC [Member]
Segment Reporting Information [Line Items]
Revenues	5,655	5,351	5,328
R&B [Member]
Segment Reporting Information [Line Items]
Revenues	3,748	3,471	3,571
Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues	9,317	8,747	8,832
Income from operations	2,378	2,116	2,181
Operating Segments [Member] | HWC [Member]
Segment Reporting Information [Line Items]
Revenues	5,582	5,287	5,268
Income from operations	1,565	1,382	1,346
Operating Segments [Member] | R&B [Member]
Segment Reporting Information [Line Items]
Revenues	3,735	3,460	3,564
Income from operations	$ 813	$ 734	$ 835