Fair Value Measurements and Disclosures - Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 30, 2017
Assets:
Total marketable securities	$ 62,501	$ 82,130
Fair Value, Measurements, Recurring [Member]
Assets:
Total marketable securities	62,502	82,130
Total	78,045	82,386
Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Total marketable securities	0	0
Total	9,563	256
Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Total marketable securities	62,502	82,130
Total	68,482	82,130
Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Total marketable securities	0	0
Total	0	0
Money market funds [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Total cash equivalents	9,563	256
Money market funds [Member] | Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Total cash equivalents	9,563	256
Money market funds [Member] | Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Total cash equivalents	0	0
Money market funds [Member] | Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Total cash equivalents	0	0
Commercial paper and Certificates of deposit [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Total cash equivalents	5,980	0
Commercial paper and Certificates of deposit [Member] | Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Total cash equivalents	0	0
Commercial paper and Certificates of deposit [Member] | Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Total cash equivalents	5,980	0
Commercial paper and Certificates of deposit [Member] | Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Total cash equivalents	0	0
U.S. Government agency debt securities [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Total marketable securities	1,499	1,495
U.S. Government agency debt securities [Member] | Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Total marketable securities	0	0
U.S. Government agency debt securities [Member] | Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Total marketable securities	1,499	1,495
U.S. Government agency debt securities [Member] | Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Total marketable securities	0	0
Certificates of deposits [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Total marketable securities	9,000	14,497
Certificates of deposits [Member] | Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Total marketable securities	0	0
Certificates of deposits [Member] | Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Total marketable securities	9,000	14,497
Certificates of deposits [Member] | Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Total marketable securities	0	0
Commercial paper [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Total marketable securities	4,974	7,949
Commercial paper [Member] | Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Total marketable securities	0	0
Commercial paper [Member] | Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Total marketable securities	4,974	7,949
Commercial paper [Member] | Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Total marketable securities	0	0
Corporate debt securities [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Total marketable securities	36,677	47,968
Corporate debt securities [Member] | Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Total marketable securities	0	0
Corporate debt securities [Member] | Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Total marketable securities	36,677	47,968
Corporate debt securities [Member] | Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Total marketable securities	0	0
Asset-backed Securities [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Total marketable securities	10,352	10,221
Asset-backed Securities [Member] | Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Total marketable securities	0	0
Asset-backed Securities [Member] | Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Total marketable securities	10,352	10,221
Asset-backed Securities [Member] | Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Total marketable securities	$ 0	$ 0