Fair Value Measurements - Schedule of Assets Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment in Persol Holdings	$ 213.6	$ 228.1
Measured on Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	4.5	4.3
Investment in Persol Holdings	213.6	228.1
Total assets at fair value	218.1	232.4
Measured on Recurring Basis | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	4.5	4.3
Investment in Persol Holdings	213.6	228.1
Total assets at fair value	218.1	232.4
Measured on Recurring Basis | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	0.0	0.0
Investment in Persol Holdings	0.0	0.0
Total assets at fair value	0.0	0.0
Measured on Recurring Basis | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	0.0	0.0
Investment in Persol Holdings	0.0	0.0
Total assets at fair value	$ 0.0	$ 0.0