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Income Taxes - Operating and capital loss carryforwards available (Details) - Dec. 31, 2017 £ in Millions, $ in Millions | USD ($) | GBP (£)
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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

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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 + Details
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