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Cash and Cash Equivalents and Short-term Investments (Details) £ in Millions, $ in Millions | 6 Months Ended |  |
--- | --- | --- | ---
Jun. 30, 2018 USD ($) | Jun. 30, 2018 GBP (£) | Jun. 30, 2018 USD ($) | Dec. 31, 2017 GBP (£) | Dec. 31, 2017 USD ($)
Cash, Cash Equivalents, and Short-term Investments [Abstract] |  |  |  |  |
Cash and cash equivalents and short-term investments |  |  | $ 660.0 |  | $ 1,285.0
Cash and cash equivalents and short term investments, period increase (decrease) | $ (625.0) |  |  |  |
Restricted cash |  |  | 99.0 |  | 96.0
Operating funds in U.K. |  | £ 42.7 | $ 56.5 | £ 42.7 | $ 57.1

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- Definition Cash And Cash Equivalents And Short Term Investments, Period Increase (Decrease) + References No definition available. + Details
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- Definition Represents cash that is restricted for minimum operating fund requirements. + References No definition available. + Details
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- Definition The current cash, cash equivalents and investments that are restricted as to withdrawal or usage. Restrictions may include legally restricted deposits held as compensating balances against short-term borrowing arrangements, contracts entered into with others, or entity statements of intention with regard to particular deposits; however, time deposits and short-term certificates of deposit are not generally included in legally restricted deposits. Excludes compensating balance arrangements that are not agreements which legally restrict the use of cash amounts shown on the balance sheet. Includes current cash equivalents and investments that are similarly restricted as to withdrawal, usage or disposal. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.1) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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