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Accounting policies - IFRS 9 and IFRS 15 Implementations - Additional Information (Detail) - GBP (£)
£ in Millions
 12 Months Ended
Jan. 01, 2018
Dec. 31, 2018
Dec. 31, 2017
Disclosure of initial application of standards or interpretations [line items]      
Work in progress reclassified as trade and other receivables   £ 366.5 £ 401.1
181 days - 1 year [member]      
Disclosure of initial application of standards or interpretations [line items]      
Percentage of trade receivables for which loss allowance required   50.00%  
Over 1 year [member]      
Disclosure of initial application of standards or interpretations [line items]      
Percentage of trade receivables for which loss allowance required   100.00%  
Retained earnings [member]      
Disclosure of initial application of standards or interpretations [line items]      
Accounting policy change (IFRS 9) £ 407.4 £ 407.4 [1],[2]  
Other reserves [member]      
Disclosure of initial application of standards or interpretations [line items]      
Accounting policy change (IFRS 9) £ (407.4) £ (407.4) [2],[3]  
[1] Retained earnings have been restated for the impact of the adoption of IFRS 15 Revenue from Contracts with Customers, as described in the accounting policies.
[2] The impact of the adoption of IFRS 9 Financial Instruments from 1 January 2018 is described in the accounting policies on pages F-2 to F-3.
[3] Other reserves are analysed in note 26.