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Accounting policies - Summary of the Impact of Adopting IFRS 15 (Detail) - GBP (£)
£ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Disclosure of initial application of standards or interpretations [line items]      
Revenue [1],[2] £ 15,602.4 £ 15,804.2 £ 14,887.3
Costs of services (12,663.5) (12,629.0) [2] (11,846.5) [2]
Gross profit £ 2,938.9 3,175.2 [2] 3,040.8 [2]
Previously stated [member]      
Disclosure of initial application of standards or interpretations [line items]      
Revenue   15,265.4 14,388.9
Costs of services   (12,090.2) (11,348.1)
Gross profit   3,175.2 3,040.8
Impact due to adoption of IFRS 15 [member]      
Disclosure of initial application of standards or interpretations [line items]      
Revenue   538.8 498.4
Costs of services   £ (538.8) £ (498.4)
[1] Intersegment sales have not been separately disclosed as they are not material.
[2] Prior year figures have been restated for the impact of the adoption of IFRS 15 Revenue from Contracts with Customers, as described in the accounting policies.