Investments in Joint Ventures and Associates - Summary of Financial Statements (Detail) - GBP (£) £ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Investments in subsidiaries, joint ventures and associates [abstract]
Associates	£ 395	£ 1,245
Joint ventures	3	2
Total	398	1,247
Associates	77	98
Joint ventures	1	(1)
Total	£ 78	£ 97	£ 68

The amount of investments in associates. [Refer: Associates [member]]

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The amount of investments in joint ventures. [Refer: Joint ventures [member]]

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The amount of investments in subsidiaries, joint ventures and associates in an entity's separate financial statements. [Refer: Associates [member]; Joint ventures [member]; Subsidiaries [member]; Investments in subsidiaries]

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The entity's share of the profit (loss) of associates accounted for using the equity method. [Refer: Associates [member]; Investments accounted for using equity method; Profit (loss)]

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The entity's share of the profit (loss) of associates and joint ventures accounted for using the equity method. [Refer: Associates [member]; Investments accounted for using equity method; Joint ventures [member]; Profit (loss)]

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The entity's share of the profit (loss) of joint ventures accounted for using the equity method. [Refer: Investments accounted for using equity method; Joint ventures [member]; Profit (loss)]

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