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Fair Value Measurements - Narrative (Details) - USD ($)
$ in Millions
Dec. 30, 2018
Sep. 30, 2018
Dec. 31, 2017
Persol Holdings Investment      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Cost of equity securities $ 18.8   $ 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 $ 5.0