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Income Taxes - Operating and capital loss carryforwards available (Details) - Dec. 31, 2017 £ in Millions, $ in Millions		USD ($)	GBP (£)
U.S. | Expiring 2018 - 2022
Operating and capital loss carryforwards [Line Items]
Operating loss carryforwards		$ 130
U.S. | Capital loss carryforward | Expiring 2022
Operating and capital loss carryforwards [Line Items]
Capital loss carryforwards		109
United Kingdom
Operating and capital loss carryforwards [Line Items]
Operating loss carryforwards	[1]	$ 252	£ 186
[1] £186 million

X

- Definition

Amount of gross capital loss carryforward, before tax effects, available and included in total capital loss carryforwards at the end of the period.

+ References

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- Definition

Amount of operating loss carryforward, before tax effects, available to reduce future taxable income under enacted tax laws.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32559-109319

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