Revenues - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Disaggregation of Revenue [Line Items]
Revenue	$ 4,744	$ 4,243	$ 13,730	$ 12,468
Time-and-materials [Member]
Disaggregation of Revenue [Line Items]
Revenue	2,342	2,201	6,865	6,413
Fixed-price [Member]
Disaggregation of Revenue [Line Items]
Revenue	1,876	1,568	5,341	4,593
Transaction or volume-based [Member]
Disaggregation of Revenue [Line Items]
Revenue	526	474	1,524	1,462
North America [Member]
Disaggregation of Revenue [Line Items]
Revenue	3,486	3,179	10,141	9,375
United Kingdom [Member]
Disaggregation of Revenue [Line Items]
Revenue	430	345	1,203	996
Europe, excluding United Kingdom [Member]
Disaggregation of Revenue [Line Items]
Revenue	484	437	1,422	1,293
Europe [Member]
Disaggregation of Revenue [Line Items]
Revenue	914	782	2,625	2,289
Rest of World [Member]
Disaggregation of Revenue [Line Items]
Revenue	344	282	964	804
Consulting And Technology Services [Member]
Disaggregation of Revenue [Line Items]
Revenue	2,990	2,642	8,585	7,646
Outsourcing Services [Member]
Disaggregation of Revenue [Line Items]
Revenue	1,754	1,601	5,145	4,822
Financial Services [Member]
Disaggregation of Revenue [Line Items]
Revenue	1,544	1,469	4,504	4,316
Financial Services [Member] | Time-and-materials [Member]
Disaggregation of Revenue [Line Items]
Revenue	922	932	2,729	2,689
Financial Services [Member] | Fixed-price [Member]
Disaggregation of Revenue [Line Items]
Revenue	527	450	1,498	1,377
Financial Services [Member] | Transaction or volume-based [Member]
Disaggregation of Revenue [Line Items]
Revenue	95	87	277	250
Financial Services [Member] | North America [Member]
Disaggregation of Revenue [Line Items]
Revenue	1,075	1,033	3,137	3,023
Financial Services [Member] | United Kingdom [Member]
Disaggregation of Revenue [Line Items]
Revenue	140	123	395	353
Financial Services [Member] | Europe, excluding United Kingdom [Member]
Disaggregation of Revenue [Line Items]
Revenue	187	181	565	554
Financial Services [Member] | Europe [Member]
Disaggregation of Revenue [Line Items]
Revenue	327	304	960	907
Financial Services [Member] | Rest of World [Member]
Disaggregation of Revenue [Line Items]
Revenue	142	132	407	386
Financial Services [Member] | Consulting And Technology Services [Member]
Disaggregation of Revenue [Line Items]
Revenue	1,049	984	3,028	2,864
Financial Services [Member] | Outsourcing Services [Member]
Disaggregation of Revenue [Line Items]
Revenue	495	485	1,476	1,452
Products and Resources [Member]
Disaggregation of Revenue [Line Items]
Revenue	1,107	927	3,160	2,748
Products and Resources [Member] | Time-and-materials [Member]
Disaggregation of Revenue [Line Items]
Revenue	468	392	1,332	1,164
Products and Resources [Member] | Fixed-price [Member]
Disaggregation of Revenue [Line Items]
Revenue	530	434	1,517	1,286
Products and Resources [Member] | Transaction or volume-based [Member]
Disaggregation of Revenue [Line Items]
Revenue	109	101	311	298
Products and Resources [Member] | North America [Member]
Disaggregation of Revenue [Line Items]
Revenue	749	666	2,190	1,975
Products and Resources [Member] | United Kingdom [Member]
Disaggregation of Revenue [Line Items]
Revenue	125	96	347	278
Products and Resources [Member] | Europe, excluding United Kingdom [Member]
Disaggregation of Revenue [Line Items]
Revenue	145	97	380	300
Products and Resources [Member] | Europe [Member]
Disaggregation of Revenue [Line Items]
Revenue	270	193	727	578
Products and Resources [Member] | Rest of World [Member]
Disaggregation of Revenue [Line Items]
Revenue	88	68	243	195
Products and Resources [Member] | Consulting And Technology Services [Member]
Disaggregation of Revenue [Line Items]
Revenue	715	564	1,999	1,674
Products and Resources [Member] | Outsourcing Services [Member]
Disaggregation of Revenue [Line Items]
Revenue	392	363	1,161	1,074
Communication, Media and Technology [Member]
Disaggregation of Revenue [Line Items]
Revenue	739	616	2,099	1,822
Communication, Media and Technology [Member] | Time-and-materials [Member]
Disaggregation of Revenue [Line Items]
Revenue	437	372	1,256	1,112
Communication, Media and Technology [Member] | Fixed-price [Member]
Disaggregation of Revenue [Line Items]
Revenue	265	218	744	638
Communication, Media and Technology [Member] | Transaction or volume-based [Member]
Disaggregation of Revenue [Line Items]
Revenue	37	26	99	72
Communication, Media and Technology [Member] | North America [Member]
Disaggregation of Revenue [Line Items]
Revenue	500	426	1,420	1,286
Communication, Media and Technology [Member] | United Kingdom [Member]
Disaggregation of Revenue [Line Items]
Revenue	121	86	332	249
Communication, Media and Technology [Member] | Europe, excluding United Kingdom [Member]
Disaggregation of Revenue [Line Items]
Revenue	34	43	121	122
Communication, Media and Technology [Member] | Europe [Member]
Disaggregation of Revenue [Line Items]
Revenue	155	129	453	371
Communication, Media and Technology [Member] | Rest of World [Member]
Disaggregation of Revenue [Line Items]
Revenue	84	61	226	165
Communication, Media and Technology [Member] | Consulting And Technology Services [Member]
Disaggregation of Revenue [Line Items]
Revenue	441	369	1,259	1,055
Communication, Media and Technology [Member] | Outsourcing Services [Member]
Disaggregation of Revenue [Line Items]
Revenue	298	247	840	767
Healthcare Segment [Member]
Disaggregation of Revenue [Line Items]
Revenue	1,354	1,231	3,967	3,582
Healthcare Segment [Member] | Time-and-materials [Member]
Disaggregation of Revenue [Line Items]
Revenue	515	505	1,548	1,448
Healthcare Segment [Member] | Fixed-price [Member]
Disaggregation of Revenue [Line Items]
Revenue	554	466	1,582	1,292
Healthcare Segment [Member] | Transaction or volume-based [Member]
Disaggregation of Revenue [Line Items]
Revenue	285	260	837	842
Healthcare Segment [Member] | North America [Member]
Disaggregation of Revenue [Line Items]
Revenue	1,162	1,054	3,394	3,091
Healthcare Segment [Member] | United Kingdom [Member]
Disaggregation of Revenue [Line Items]
Revenue	44	40	129	116
Healthcare Segment [Member] | Europe, excluding United Kingdom [Member]
Disaggregation of Revenue [Line Items]
Revenue	118	116	356	317
Healthcare Segment [Member] | Europe [Member]
Disaggregation of Revenue [Line Items]
Revenue	162	156	485	433
Healthcare Segment [Member] | Rest of World [Member]
Disaggregation of Revenue [Line Items]
Revenue	30	21	88	58
Healthcare Segment [Member] | Consulting And Technology Services [Member]
Disaggregation of Revenue [Line Items]
Revenue	785	725	2,299	2,053
Healthcare Segment [Member] | Outsourcing Services [Member]
Disaggregation of Revenue [Line Items]
Revenue	$ 569	$ 506	$ 1,668	$ 1,529