Marketable Securities - Letters of Credit (Details) - USD ($)	Dec. 30, 2018	Dec. 31, 2017
Short-term Debt [Line Items]
Total amount of letters of credit	$ 48,800,000	$ 56,000,000
Amount outstanding, letters of credit	0
Letter of Credit [Member]
Short-term Debt [Line Items]
Fair market value of letters of credit	$ 54,200,000	$ 63,000,000