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Fair Value Measurements - Narrative (Details) - USD ($) $ in Millions | Sep. 30, 2018 | Dec. 31, 2017
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Persol Holdings Investment |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Cost of equity securities | $ 18.2 | $ 18.4
Business Talent Group, LLC |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Equity Securities without Readily Determinable Fair Value, Amount | $ 5.0 |

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