Revenues and Trade Accounts Receivable - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Disaggregation of Revenue [Line Items]
Revenue	$ 5,415	$ 5,044	$ 15,775	$ 14,654
Time-and-materials
Disaggregation of Revenue [Line Items]
Revenue	2,370	2,282	6,946	6,671
Fixed-price
Disaggregation of Revenue [Line Items]
Revenue	2,547	2,300	7,370	6,669
Transaction or volume-based
Disaggregation of Revenue [Line Items]
Revenue	498	462	1,459	1,314
North America
Disaggregation of Revenue [Line Items]
Revenue	4,028	3,735	11,794	10,876
United Kingdom
Disaggregation of Revenue [Line Items]
Revenue	503	482	1,442	1,382
Europe, excluding United Kingdom
Disaggregation of Revenue [Line Items]
Revenue	539	485	1,552	1,438
Europe
Disaggregation of Revenue [Line Items]
Revenue	1,042	967	2,994	2,820
Rest of World
Disaggregation of Revenue [Line Items]
Revenue	345	342	987	958
Consulting And Technology Services
Disaggregation of Revenue [Line Items]
Revenue	3,494	3,226	10,109	9,255
Outsourcing Services
Disaggregation of Revenue [Line Items]
Revenue	1,921	1,818	5,666	5,399
Healthcare Segment
Disaggregation of Revenue [Line Items]
Revenue	1,604	1,514	4,726	4,391
Healthcare Segment \| Time-and-materials
Disaggregation of Revenue [Line Items]
Revenue	506	499	1,482	1,486
Healthcare Segment \| Fixed-price
Disaggregation of Revenue [Line Items]
Revenue	791	738	2,332	2,111
Healthcare Segment \| Transaction or volume-based
Disaggregation of Revenue [Line Items]
Revenue	307	277	912	794
Healthcare Segment \| North America
Disaggregation of Revenue [Line Items]
Revenue	1,331	1,295	3,959	3,756
Healthcare Segment \| United Kingdom
Disaggregation of Revenue [Line Items]
Revenue	56	47	156	138
Healthcare Segment \| Europe, excluding United Kingdom
Disaggregation of Revenue [Line Items]
Revenue	174	143	500	413
Healthcare Segment \| Europe
Disaggregation of Revenue [Line Items]
Revenue	230	190	656	551
Healthcare Segment \| Rest of World
Disaggregation of Revenue [Line Items]
Revenue	43	29	111	84
Healthcare Segment \| Consulting And Technology Services
Disaggregation of Revenue [Line Items]
Revenue	929	895	2,707	2,560
Healthcare Segment \| Outsourcing Services
Disaggregation of Revenue [Line Items]
Revenue	675	619	2,019	1,831
Financial Services
Disaggregation of Revenue [Line Items]
Revenue	1,578	1,486	4,587	4,318
Financial Services \| Time-and-materials
Disaggregation of Revenue [Line Items]
Revenue	829	833	2,405	2,421
Financial Services \| Fixed-price
Disaggregation of Revenue [Line Items]
Revenue	697	608	2,035	1,761
Financial Services \| Transaction or volume-based
Disaggregation of Revenue [Line Items]
Revenue	52	45	147	136
Financial Services \| North America
Disaggregation of Revenue [Line Items]
Revenue	1,115	1,048	3,254	3,064
Financial Services \| United Kingdom
Disaggregation of Revenue [Line Items]
Revenue	166	147	478	428
Financial Services \| Europe, excluding United Kingdom
Disaggregation of Revenue [Line Items]
Revenue	164	156	471	457
Financial Services \| Europe
Disaggregation of Revenue [Line Items]
Revenue	330	303	949	885
Financial Services \| Rest of World
Disaggregation of Revenue [Line Items]
Revenue	133	135	384	369
Financial Services \| Consulting And Technology Services
Disaggregation of Revenue [Line Items]
Revenue	1,118	1,051	3,237	3,025
Financial Services \| Outsourcing Services
Disaggregation of Revenue [Line Items]
Revenue	460	435	1,350	1,293
Products and Resources
Disaggregation of Revenue [Line Items]
Revenue	1,383	1,228	3,967	3,487
Products and Resources \| Time-and-materials
Disaggregation of Revenue [Line Items]
Revenue	589	511	1,725	1,428
Products and Resources \| Fixed-price
Disaggregation of Revenue [Line Items]
Revenue	704	627	1,976	1,803
Products and Resources \| Transaction or volume-based
Disaggregation of Revenue [Line Items]
Revenue	90	90	266	256
Products and Resources \| North America
Disaggregation of Revenue [Line Items]
Revenue	978	828	2,807	2,362
Products and Resources \| United Kingdom
Disaggregation of Revenue [Line Items]
Revenue	156	157	441	415
Products and Resources \| Europe, excluding United Kingdom
Disaggregation of Revenue [Line Items]
Revenue	167	154	479	447
Products and Resources \| Europe
Disaggregation of Revenue [Line Items]
Revenue	323	311	920	862
Products and Resources \| Rest of World
Disaggregation of Revenue [Line Items]
Revenue	82	89	240	263
Products and Resources \| Consulting And Technology Services
Disaggregation of Revenue [Line Items]
Revenue	981	833	2,786	2,297
Products and Resources \| Outsourcing Services
Disaggregation of Revenue [Line Items]
Revenue	402	395	1,181	1,190
Communication, Media and Technology
Disaggregation of Revenue [Line Items]
Revenue	850	816	2,495	2,458
Communication, Media and Technology \| Time-and-materials
Disaggregation of Revenue [Line Items]
Revenue	446	439	1,334	1,336
Communication, Media and Technology \| Fixed-price
Disaggregation of Revenue [Line Items]
Revenue	355	327	1,027	994
Communication, Media and Technology \| Transaction or volume-based
Disaggregation of Revenue [Line Items]
Revenue	49	50	134	128
Communication, Media and Technology \| North America
Disaggregation of Revenue [Line Items]
Revenue	604	564	1,774	1,694
Communication, Media and Technology \| United Kingdom
Disaggregation of Revenue [Line Items]
Revenue	125	131	367	401
Communication, Media and Technology \| Europe, excluding United Kingdom
Disaggregation of Revenue [Line Items]
Revenue	34	32	102	121
Communication, Media and Technology \| Europe
Disaggregation of Revenue [Line Items]
Revenue	159	163	469	522
Communication, Media and Technology \| Rest of World
Disaggregation of Revenue [Line Items]
Revenue	87	89	252	242
Communication, Media and Technology \| Consulting And Technology Services
Disaggregation of Revenue [Line Items]
Revenue	466	447	1,379	1,373
Communication, Media and Technology \| Outsourcing Services
Disaggregation of Revenue [Line Items]
Revenue	$ 384	$ 369	$ 1,116	$ 1,085