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Accounting policies - Summary of the Impact of Adopting IFRS 15 (Detail) - GBP (£)
£ in Millions
 6 Months Ended 12 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Disclosure of initial application of standards or interpretations [line items]      
Revenue £ 7,492.7 £ 7,650.4 [1],[2] £ 15,804.2 [1],[2]
Costs of services (6,218.7) (6,282.3) [1],[2] (12,629.0) [1],[2]
Gross profit £ 1,274.0 1,368.1 [1] 3,175.2 [1]
Pre IFRS 15 [Member]      
Disclosure of initial application of standards or interpretations [line items]      
Revenue   7,403.6 15,265.4
Costs of services   (6,035.5) (12,090.2)
Gross profit   1,368.1 3,175.2
Impact due to adoption of IFRS 15 [member]      
Disclosure of initial application of standards or interpretations [line items]      
Revenue   246.8 538.8
Costs of services   £ (246.8) £ (538.8)
[1] Prior year figures have been restated for the impact of the adoption of IFRS 15: Revenue from Contracts with Customers and the change in income statement presentation, as described in note 2.
[2] Prior year figures have been restated for the impact of the adoption of IFRS 15: Revenue from Contracts with Customers, as described in note 2.