Cash Equivalents and Investments (Available for sales securities) (Details) - USD ($) $ in Millions		Jun. 29, 2018	Dec. 29, 2017
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Debt Securities, Fair Value		$ 0.0	$ 205.7
Cash Equivalents			26.8
Short-term investments		0.0	178.9	[1]
Total available-for-sale securities			205.7
US Treasury Securities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Debt Securities, Fair Value	[2]	0.0	9.6
Corporate Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Debt Securities, Fair Value	[2]	0.0	96.0
Commercial Paper
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Debt Securities, Fair Value	[2]	$ 0.0	$ 100.1

[1] See Note 2 for a summary of adjustments

[2] The Company’s available-for sale securities are valued using readily available pricing sources for comparable instruments, or model-driven valuations using significant inputs derived from or corroborated by observable market data, including yield curves and credit ratings.