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Investments in Joint Ventures and Associates - Summary of Financial Statements (Detail) - GBP (£)
£ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Investments in subsidiaries, joint ventures and associates [abstract]      
Associates £ 392 £ 395  
Joint ventures   3  
Total 392 398  
Associates 43 77  
Joint ventures 1 1  
Total £ 44 £ 78 £ 97