Fair Value Measurements - Narrative (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended	9 Months Ended
	Sep. 27, 2020	Mar. 29, 2020	Sep. 29, 2019	Jun. 28, 2020	Sep. 27, 2020	Sep. 29, 2019	Dec. 29, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Indemnification liabilities, expense	$ 2.5				$ 2.5
Expense related to earnout liability	0.5				0.5
Goodwill impairment charge	0.0	$ 147.7	$ 0.0	$ 147.7	147.7	$ 0.0
Persol Holdings Investment
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cost of equity securities	19.6				19.6		$ 18.9
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		5.0
Kenzie Academy Inc.
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities without readily determinable fair value, with changes in fair value in net income	1.4				1.4		1.3
Other Assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	5.3				5.3		$ 4.9
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Indemnification liability	2.5				2.5
Contingent consideration, liability	$ 2.1				$ 2.1