Marketable Securities - Continuous Loss Position (Details) - USD ($)	12 Months Ended
	Dec. 27, 2020	Dec. 29, 2019
Long-term AFS securities
Other than temporary impairment, loss	$ 0	$ 0
Minimum | Short-term Marketable Securities
Long-term AFS securities
Remaining maturities on short-term and long-term marketable securities	1 month
Minimum | Long-term Marketable Securities
Long-term AFS securities
Remaining maturities on short-term and long-term marketable securities		13 months
Maximum | Short-term Marketable Securities
Long-term AFS securities
Remaining maturities on short-term and long-term marketable securities	12 months
Maximum | Long-term Marketable Securities
Long-term AFS securities
Remaining maturities on short-term and long-term marketable securities		36 months
Corporate debt securities
Short-term AFS securities
Fair value, less than 12 months, short-term AFS securities	$ 33,735,000	$ 20,975,000
Gross unrealized losses, less than 12 months, short-term AFS securities	(8,000)	(6,000)
Fair value, 12 months or greater, short-term AFS securities	0	8,251,000
Gross unrealized losses, 12 months or greater, short-term AFS securities	0	(3,000)
Fair value, short-term AFS securities	33,735,000	29,226,000
Gross unrealized losses, short-term AFS securities	(8,000)	(9,000)
Long-term AFS securities
Fair value, less than 12 months, long-term AFS securities	6,717,000	35,891,000
Gross unrealized losses, less than 12 months, long-term AFS securities	(5,000)	(25,000)
Fair value, 12 months or greater, long-term AFS securities	0	4,502,000
Gross unrealized losses, 12 months or greater, long-term AFS securities	0	(4,000)
Fair value, long-term AFS securities	6,717,000	40,393,000
Gross unrealized losses, long-term AFS securities	(5,000)	(29,000)
U.S. Treasury securities
Short-term AFS securities
Fair value, less than 12 months, short-term AFS securities	20,133,000	13,296,000
Gross unrealized losses, less than 12 months, short-term AFS securities	(3,000)	(3,000)
Fair value, 12 months or greater, short-term AFS securities	0	11,147,000
Gross unrealized losses, 12 months or greater, short-term AFS securities	0	(8,000)
Fair value, short-term AFS securities	20,133,000	24,443,000
Gross unrealized losses, short-term AFS securities	(3,000)	(11,000)
Long-term AFS securities
Fair value, less than 12 months, long-term AFS securities		60,935,000
Gross unrealized losses, less than 12 months, long-term AFS securities		(103,000)
Fair value, 12 months or greater, long-term AFS securities		0
Gross unrealized losses, 12 months or greater, long-term AFS securities		0
Fair value, long-term AFS securities		60,935,000
Gross unrealized losses, long-term AFS securities		(103,000)
U.S. governmental agency securities
Short-term AFS securities
Fair value, less than 12 months, short-term AFS securities	4,999,000	0
Gross unrealized losses, less than 12 months, short-term AFS securities	(2,000)	0
Fair value, 12 months or greater, short-term AFS securities	8,749,000	15,000,000
Gross unrealized losses, 12 months or greater, short-term AFS securities	(1,000)	(4,000)
Fair value, short-term AFS securities	13,748,000	15,000,000
Gross unrealized losses, short-term AFS securities	(3,000)	(4,000)
Long-term AFS securities
Fair value, less than 12 months, long-term AFS securities	26,236,000	34,167,000
Gross unrealized losses, less than 12 months, long-term AFS securities	(13,000)	(84,000)
Fair value, 12 months or greater, long-term AFS securities	0	0
Gross unrealized losses, 12 months or greater, long-term AFS securities	0	0
Fair value, long-term AFS securities	26,236,000	34,167,000
Gross unrealized losses, long-term AFS securities	(13,000)	(84,000)
Municipal securities
Long-term AFS securities
Fair value, less than 12 months, long-term AFS securities	4,984,000
Gross unrealized losses, less than 12 months, long-term AFS securities	(10,000)
Fair value, 12 months or greater, long-term AFS securities	0
Gross unrealized losses, 12 months or greater, long-term AFS securities	0
Fair value, long-term AFS securities	4,984,000
Gross unrealized losses, long-term AFS securities	(10,000)
Total short-term AFS securities
Short-term AFS securities
Fair value, less than 12 months, short-term AFS securities	58,867,000	34,271,000
Gross unrealized losses, less than 12 months, short-term AFS securities	(13,000)	(9,000)
Fair value, 12 months or greater, short-term AFS securities	8,749,000	34,398,000
Gross unrealized losses, 12 months or greater, short-term AFS securities	(1,000)	(15,000)
Fair value, short-term AFS securities	67,616,000	68,669,000
Gross unrealized losses, short-term AFS securities	(14,000)	(24,000)
Total long-term AFS securities
Long-term AFS securities
Fair value, less than 12 months, long-term AFS securities	37,937,000	130,993,000
Gross unrealized losses, less than 12 months, long-term AFS securities	(28,000)	(212,000)
Fair value, 12 months or greater, long-term AFS securities	0	4,502,000
Gross unrealized losses, 12 months or greater, long-term AFS securities	0	(4,000)
Fair value, long-term AFS securities	37,937,000	135,495,000
Gross unrealized losses, long-term AFS securities	$ (28,000)	$ (216,000)