Revenues - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Disaggregation of Revenue [Line Items]
Revenue	$ 4,585	$ 4,000	$ 8,986	$ 8,225
Time-and-materials [Member]
Disaggregation of Revenue [Line Items]
Revenue	2,290	2,061	4,523	4,212
Fixed-price [Member]
Disaggregation of Revenue [Line Items]
Revenue	1,784	1,471	3,465	3,025
Transaction or volume-based [Member]
Disaggregation of Revenue [Line Items]
Revenue	511	468	998	988
North America [Member]
Disaggregation of Revenue [Line Items]
Revenue	3,372	3,006	6,655	6,196
United Kingdom [Member]
Disaggregation of Revenue [Line Items]
Revenue	403	314	773	651
Europe, excluding United Kingdom [Member]
Disaggregation of Revenue [Line Items]
Revenue	482	419	938	856
Europe [Member]
Disaggregation of Revenue [Line Items]
Revenue	885	733	1,711	1,507
Rest of World [Member]
Disaggregation of Revenue [Line Items]
Revenue	328	261	620	522
Consulting And Technology Services [Member]
Disaggregation of Revenue [Line Items]
Revenue	2,871	2,457	5,595	5,004
Outsourcing Services [Member]
Disaggregation of Revenue [Line Items]
Revenue	1,714	1,543	3,391	3,221
Financial Services [Member]
Disaggregation of Revenue [Line Items]
Revenue	1,502	1,396	2,960	2,847
Financial Services [Member] | Time-and-materials [Member]
Disaggregation of Revenue [Line Items]
Revenue	908	873	1,807	1,757
Financial Services [Member] | Fixed-price [Member]
Disaggregation of Revenue [Line Items]
Revenue	500	444	971	927
Financial Services [Member] | Transaction or volume-based [Member]
Disaggregation of Revenue [Line Items]
Revenue	94	79	182	163
Financial Services [Member] | North America [Member]
Disaggregation of Revenue [Line Items]
Revenue	1,049	978	2,062	1,990
Financial Services [Member] | United Kingdom [Member]
Disaggregation of Revenue [Line Items]
Revenue	130	110	255	230
Financial Services [Member] | Europe, excluding United Kingdom [Member]
Disaggregation of Revenue [Line Items]
Revenue	186	182	378	373
Financial Services [Member] | Europe [Member]
Disaggregation of Revenue [Line Items]
Revenue	316	292	633	603
Financial Services [Member] | Rest of World [Member]
Disaggregation of Revenue [Line Items]
Revenue	137	126	265	254
Financial Services [Member] | Consulting And Technology Services [Member]
Disaggregation of Revenue [Line Items]
Revenue	1,012	933	1,979	1,880
Financial Services [Member] | Outsourcing Services [Member]
Disaggregation of Revenue [Line Items]
Revenue	490	463	981	967
Products and Resources [Member]
Disaggregation of Revenue [Line Items]
Revenue	1,055	867	2,053	1,821
Products and Resources [Member] | Time-and-materials [Member]
Disaggregation of Revenue [Line Items]
Revenue	446	363	864	772
Products and Resources [Member] | Fixed-price [Member]
Disaggregation of Revenue [Line Items]
Revenue	506	409	987	852
Products and Resources [Member] | Transaction or volume-based [Member]
Disaggregation of Revenue [Line Items]
Revenue	103	95	202	197
Products and Resources [Member] | North America [Member]
Disaggregation of Revenue [Line Items]
Revenue	723	620	1,441	1,309
Products and Resources [Member] | United Kingdom [Member]
Disaggregation of Revenue [Line Items]
Revenue	116	89	222	182
Products and Resources [Member] | Europe, excluding United Kingdom [Member]
Disaggregation of Revenue [Line Items]
Revenue	132	94	235	203
Products and Resources [Member] | Europe [Member]
Disaggregation of Revenue [Line Items]
Revenue	248	183	457	385
Products and Resources [Member] | Rest of World [Member]
Disaggregation of Revenue [Line Items]
Revenue	84	64	155	127
Products and Resources [Member] | Consulting And Technology Services [Member]
Disaggregation of Revenue [Line Items]
Revenue	668	520	1,284	1,110
Products and Resources [Member] | Outsourcing Services [Member]
Disaggregation of Revenue [Line Items]
Revenue	387	347	769	711
Communication, Media and Technology [Member]
Disaggregation of Revenue [Line Items]
Revenue	703	580	1,360	1,206
Communication, Media and Technology [Member] | Time-and-materials [Member]
Disaggregation of Revenue [Line Items]
Revenue	422	357	819	740
Communication, Media and Technology [Member] | Fixed-price [Member]
Disaggregation of Revenue [Line Items]
Revenue	249	201	479	420
Communication, Media and Technology [Member] | Transaction or volume-based [Member]
Disaggregation of Revenue [Line Items]
Revenue	32	22	62	46
Communication, Media and Technology [Member] | North America [Member]
Disaggregation of Revenue [Line Items]
Revenue	469	409	920	860
Communication, Media and Technology [Member] | United Kingdom [Member]
Disaggregation of Revenue [Line Items]
Revenue	112	79	211	163
Communication, Media and Technology [Member] | Europe, excluding United Kingdom [Member]
Disaggregation of Revenue [Line Items]
Revenue	44	41	87	79
Communication, Media and Technology [Member] | Europe [Member]
Disaggregation of Revenue [Line Items]
Revenue	156	120	298	242
Communication, Media and Technology [Member] | Rest of World [Member]
Disaggregation of Revenue [Line Items]
Revenue	78	51	142	104
Communication, Media and Technology [Member] | Consulting And Technology Services [Member]
Disaggregation of Revenue [Line Items]
Revenue	422	338	818	686
Communication, Media and Technology [Member] | Outsourcing Services [Member]
Disaggregation of Revenue [Line Items]
Revenue	281	242	542	520
Healthcare Segment [Member]
Disaggregation of Revenue [Line Items]
Revenue	1,325	1,157	2,613	2,351
Healthcare Segment [Member] | Time-and-materials [Member]
Disaggregation of Revenue [Line Items]
Revenue	514	468	1,033	943
Healthcare Segment [Member] | Fixed-price [Member]
Disaggregation of Revenue [Line Items]
Revenue	529	417	1,028	826
Healthcare Segment [Member] | Transaction or volume-based [Member]
Disaggregation of Revenue [Line Items]
Revenue	282	272	552	582
Healthcare Segment [Member] | North America [Member]
Disaggregation of Revenue [Line Items]
Revenue	1,131	999	2,232	2,037
Healthcare Segment [Member] | United Kingdom [Member]
Disaggregation of Revenue [Line Items]
Revenue	45	36	85	76
Healthcare Segment [Member] | Europe, excluding United Kingdom [Member]
Disaggregation of Revenue [Line Items]
Revenue	120	102	238	201
Healthcare Segment [Member] | Europe [Member]
Disaggregation of Revenue [Line Items]
Revenue	165	138	323	277
Healthcare Segment [Member] | Rest of World [Member]
Disaggregation of Revenue [Line Items]
Revenue	29	20	58	37
Healthcare Segment [Member] | Consulting And Technology Services [Member]
Disaggregation of Revenue [Line Items]
Revenue	769	666	1,514	1,328
Healthcare Segment [Member] | Outsourcing Services [Member]
Disaggregation of Revenue [Line Items]
Revenue	$ 556	$ 491	$ 1,099	$ 1,023