Fair Value of Financial Assets and Liabilities (Detail) - USD ($) $ in Thousands	Sep. 28, 2019	Dec. 29, 2018
Cash equivalents:
Money market funds	$ 12,126	$ 13,586
Marketable securities:
Marketable securities	67,865
Cash equivalents and marketable securities	79,991
Cash Equivalents, at Carrying Value		13,586
Liabilities:
Preferred stock warrant liability		898
Level 1 [Member]
Cash equivalents:
Money market funds	12,126	13,586
Marketable securities:
Cash equivalents and marketable securities	12,126
Cash Equivalents, at Carrying Value		13,586
Level 2 [Member]
Marketable securities:
Cash equivalents and marketable securities	67,865
Level 3 [Member]
Liabilities:
Preferred stock warrant liability		$ 898
U.S. Treasury Securities [Member]
Marketable securities:
Marketable securities	12,221
U.S. Treasury Securities [Member] | Level 2 [Member]
Marketable securities:
Marketable securities	12,221
U.S. Government Agency Bonds [Member]
Marketable securities:
Marketable securities	55,644
U.S. Government Agency Bonds [Member] | Level 2 [Member]
Marketable securities:
Marketable securities	$ 55,644