Fair Value Measurements - Fair Value Measurements on a Recurring Basis (Details) - USD ($) $ in Millions	Dec. 31, 2017	Jan. 01, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale investment	$ 228.1	$ 141.2
Measured on a recurring basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	4.3	4.0
Available-for-sale investment	228.1	141.2
Total assets at fair value	232.4	145.2
Measured on a recurring basis | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	4.3	4.0
Available-for-sale investment	228.1	141.2
Total assets at fair value	232.4	145.2
Measured on a recurring basis | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	0.0	0.0
Available-for-sale investment	0.0	0.0
Total assets at fair value	0.0	0.0
Measured on a recurring basis | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	0.0	0.0
Available-for-sale investment	0.0	0.0
Total assets at fair value	$ 0.0	$ 0.0