Revenues - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Disaggregation of Revenue [Line Items]
Revenue	$ 4,110	$ 3,912
Time-and-materials [Member]
Disaggregation of Revenue [Line Items]
Revenue	2,152	2,058
Fixed-price [Member]
Disaggregation of Revenue [Line Items]
Revenue	1,468	1,522
Transaction or volume-based [Member]
Disaggregation of Revenue [Line Items]
Revenue	490	332
North America [Member]
Disaggregation of Revenue [Line Items]
Revenue	3,123	2,975
United Kingdom [Member]
Disaggregation of Revenue [Line Items]
Revenue	329	310
Europe, excluding United Kingdom [Member]
Disaggregation of Revenue [Line Items]
Revenue	405	374
Europe [Member]
Disaggregation of Revenue [Line Items]
Revenue	734	684
Rest of World [Member]
Disaggregation of Revenue [Line Items]
Revenue	253	253
Consulting And Technology Services [Member]
Disaggregation of Revenue [Line Items]
Revenue	2,409	2,268
Outsourcing Services [Member]
Disaggregation of Revenue [Line Items]
Revenue	1,701	1,644
Financial Services [Member]
Disaggregation of Revenue [Line Items]
Revenue	1,436	1,461
Financial Services [Member] | Time-and-materials [Member]
Disaggregation of Revenue [Line Items]
Revenue	919	935
Financial Services [Member] | Fixed-price [Member]
Disaggregation of Revenue [Line Items]
Revenue	464	471
Financial Services [Member] | Transaction or volume-based [Member]
Disaggregation of Revenue [Line Items]
Revenue	53	55
Financial Services [Member] | North America [Member]
Disaggregation of Revenue [Line Items]
Revenue	1,018	1,044
Financial Services [Member] | United Kingdom [Member]
Disaggregation of Revenue [Line Items]
Revenue	129	116
Financial Services [Member] | Europe, excluding United Kingdom [Member]
Disaggregation of Revenue [Line Items]
Revenue	162	162
Financial Services [Member] | Europe [Member]
Disaggregation of Revenue [Line Items]
Revenue	291	278
Financial Services [Member] | Rest of World [Member]
Disaggregation of Revenue [Line Items]
Revenue	127	139
Financial Services [Member] | Consulting And Technology Services [Member]
Disaggregation of Revenue [Line Items]
Revenue	913	871
Financial Services [Member] | Outsourcing Services [Member]
Disaggregation of Revenue [Line Items]
Revenue	523	590
Products and Resources [Member]
Disaggregation of Revenue [Line Items]
Revenue	914	821
Products and Resources [Member] | Time-and-materials [Member]
Disaggregation of Revenue [Line Items]
Revenue	400	369
Products and Resources [Member] | Fixed-price [Member]
Disaggregation of Revenue [Line Items]
Revenue	414	361
Products and Resources [Member] | Transaction or volume-based [Member]
Disaggregation of Revenue [Line Items]
Revenue	100	91
Products and Resources [Member] | North America [Member]
Disaggregation of Revenue [Line Items]
Revenue	641	572
Products and Resources [Member] | United Kingdom [Member]
Disaggregation of Revenue [Line Items]
Revenue	94	87
Products and Resources [Member] | Europe, excluding United Kingdom [Member]
Disaggregation of Revenue [Line Items]
Revenue	115	109
Products and Resources [Member] | Europe [Member]
Disaggregation of Revenue [Line Items]
Revenue	209	196
Products and Resources [Member] | Rest of World [Member]
Disaggregation of Revenue [Line Items]
Revenue	64	53
Products and Resources [Member] | Consulting And Technology Services [Member]
Disaggregation of Revenue [Line Items]
Revenue	552	481
Products and Resources [Member] | Outsourcing Services [Member]
Disaggregation of Revenue [Line Items]
Revenue	362	340
Communication, Media and Technology [Member]
Disaggregation of Revenue [Line Items]
Revenue	595	509
Communication, Media and Technology [Member] | Time-and-materials [Member]
Disaggregation of Revenue [Line Items]
Revenue	375	306
Communication, Media and Technology [Member] | Fixed-price [Member]
Disaggregation of Revenue [Line Items]
Revenue	190	179
Communication, Media and Technology [Member] | Transaction or volume-based [Member]
Disaggregation of Revenue [Line Items]
Revenue	30	24
Communication, Media and Technology [Member] | North America [Member]
Disaggregation of Revenue [Line Items]
Revenue	422	336
Communication, Media and Technology [Member] | United Kingdom [Member]
Disaggregation of Revenue [Line Items]
Revenue	81	84
Communication, Media and Technology [Member] | Europe, excluding United Kingdom [Member]
Disaggregation of Revenue [Line Items]
Revenue	46	42
Communication, Media and Technology [Member] | Europe [Member]
Disaggregation of Revenue [Line Items]
Revenue	127	126
Communication, Media and Technology [Member] | Rest of World [Member]
Disaggregation of Revenue [Line Items]
Revenue	46	47
Communication, Media and Technology [Member] | Consulting And Technology Services [Member]
Disaggregation of Revenue [Line Items]
Revenue	306	278
Communication, Media and Technology [Member] | Outsourcing Services [Member]
Disaggregation of Revenue [Line Items]
Revenue	289	231
Healthcare Segment [Member]
Disaggregation of Revenue [Line Items]
Revenue	1,165	1,121
Healthcare Segment [Member] | Time-and-materials [Member]
Disaggregation of Revenue [Line Items]
Revenue	458	448
Healthcare Segment [Member] | Fixed-price [Member]
Disaggregation of Revenue [Line Items]
Revenue	400	511
Healthcare Segment [Member] | Transaction or volume-based [Member]
Disaggregation of Revenue [Line Items]
Revenue	307	162
Healthcare Segment [Member] | North America [Member]
Disaggregation of Revenue [Line Items]
Revenue	1,042	1,023
Healthcare Segment [Member] | United Kingdom [Member]
Disaggregation of Revenue [Line Items]
Revenue	25	23
Healthcare Segment [Member] | Europe, excluding United Kingdom [Member]
Disaggregation of Revenue [Line Items]
Revenue	82	61
Healthcare Segment [Member] | Europe [Member]
Disaggregation of Revenue [Line Items]
Revenue	107	84
Healthcare Segment [Member] | Rest of World [Member]
Disaggregation of Revenue [Line Items]
Revenue	16	14
Healthcare Segment [Member] | Consulting And Technology Services [Member]
Disaggregation of Revenue [Line Items]
Revenue	638	638
Healthcare Segment [Member] | Outsourcing Services [Member]
Disaggregation of Revenue [Line Items]
Revenue	$ 527	$ 483