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Acquisitions Narrative (Details) $ in Millions | 12 Months Ended
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Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($)
Goodwill and intangible assets related to acquisitions | $ 62.0 | $ 149.0 | $ 61.0
Number of businesses acquired | 10 | 10 | 5
IAN |  |  |
Number of businesses acquired | 8 | 3 | 4
CMG |  |  |
Number of businesses acquired | 2 | 7 | 1

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