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Significant Accounting Policies (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Cost method investments | $ 14.4 | $ 14.2 |
Available-for-sale securities | 1.4 | 4.3 |
Net foreign currency transaction gains (losses) | $ (7.8) | $ 12.7 | $ 4.7
Percentage of revenue from largest client | 3.00% |  |
Minimum [Member] |  |  |
Identifiable intangible assets, useful life | 5 years |  |
Maximum [Member] |  |  |
Identifiable intangible assets, useful life | 12 years |  |
Furniture [Member] | Minimum [Member] |  |  |
Property and Equipment, useful life | 7 years |  |
Furniture [Member] | Maximum [Member] |  |  |
Property and Equipment, useful life | 10 years |  |
Equipment [Member] | Minimum [Member] |  |  |
Property and Equipment, useful life | 3 years |  |
Equipment [Member] | Maximum [Member] |  |  |
Property and Equipment, useful life | 5 years |  |

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- Definition Description of risks that arise due to the volume of business transacted with a particular customer. At a minimum, the description informs financial statement users of the general nature of the risk, but excludes "Information about Major Customers" that may be disclosed elsewhere (for instance, segment disclosures). + References No definition available. + Details
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