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Discontinued Operations (Details) £ in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Jan. 10, 2018 USD ($)	Jan. 10, 2018 GBP (£)	May 11, 2017 USD ($)
Discontinued Operations, Held-for-sale or Disposed of by Sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Income from discontinued operations		$ 19
Income from discontinued operations, tax	$ (7)	$ (7)
Discontinued Operations, Disposed of by Sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Consideration from sale of business			$ 50	£ 37	$ 17
Discontinued Operations, Held-for-sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Discontinued operation, corresponding assets	$ 50