Segment Information - Revenue (Net of Reimbursable Expenses) of the Reported Segments (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Segment Reporting Information [Line Items]
Total revenue	$ 2,223	$ 2,341
Depreciation	54	59
Income from operations	432	280
HWC [Member]
Segment Reporting Information [Line Items]
Total revenue	1,182	1,353
R&B [Member]
Segment Reporting Information [Line Items]
Total revenue	1,030	981
Operating Segments [Member]
Segment Reporting Information [Line Items]
Segment revenue excluding interest income	2,163	2,277
Interest income	29	37
Total revenue	2,192	2,314
Other segment expense	1,614	1,732
Depreciation	41	43
Total segment expense	1,655	1,775
Income from operations	537	539
Operating Segments [Member] | HWC [Member]
Segment Reporting Information [Line Items]
Segment revenue excluding interest income	1,158	1,327
Interest income	7	9
Total revenue	1,165	1,336
Other segment expense	823	969
Depreciation	31	31
Total segment expense	854	1,000
Income from operations	311	336
Operating Segments [Member] | R&B [Member]
Segment Reporting Information [Line Items]
Segment revenue excluding interest income	1,005	950
Interest income	22	28
Total revenue	1,027	978
Other segment expense	791	763
Depreciation	10	12
Total segment expense	801	775
Income from operations	$ 226	$ 203