Fair Value Measurements and Disclosures - Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Apr. 01, 2017	Dec. 31, 2016
Assets:
Total marketable securities	$ 84,679	$ 82,899
Fair Value, Measurements, Recurring [Member]
Assets:
Total cash equivalents	1,858	3,458
Total marketable securities	84,679	82,899
Total	86,537	86,357
Money market funds [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Total cash equivalents	858	959
Commercial paper and corporate debt securities [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Total cash equivalents	1,000	2,499
U.S. Treasury securities and U.S. Government agency debt securities [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Total marketable securities	11,709	17,072
Certificates of deposits [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Total marketable securities	21,505	23,019
Commercial paper [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Total marketable securities	22,323	22,402
Municipal securities and corporate debt securities [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Total marketable securities	25,869	14,943
Asset-backed Securities [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Total marketable securities	3,273	5,463
Cash [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Total cash equivalents	45,600	43,600
Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Total cash equivalents	858	959
Total marketable securities	0	0
Total	858	959
Level 1 [Member] | Money market funds [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Total cash equivalents	858	959
Level 1 [Member] | Commercial paper and corporate debt securities [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Total cash equivalents	0	0
Level 1 [Member] | U.S. Treasury securities and U.S. Government agency debt securities [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Total marketable securities	0	0
Level 1 [Member] | Certificates of deposits [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Total marketable securities	0	0
Level 1 [Member] | Commercial paper [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Total marketable securities	0	0
Level 1 [Member] | Municipal securities and corporate debt securities [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Total marketable securities	0	0
Level 1 [Member] | Asset-backed Securities [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Total marketable securities	0	0
Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Total cash equivalents	1,000	2,499
Total marketable securities	84,679	82,899
Total	85,679	85,398
Level 2 [Member] | Money market funds [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Total cash equivalents	0	0
Level 2 [Member] | Commercial paper and corporate debt securities [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Total cash equivalents	1,000	2,499
Level 2 [Member] | U.S. Treasury securities and U.S. Government agency debt securities [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Total marketable securities	11,709	17,072
Level 2 [Member] | Certificates of deposits [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Total marketable securities	21,505	23,019
Level 2 [Member] | Commercial paper [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Total marketable securities	22,323	22,402
Level 2 [Member] | Municipal securities and corporate debt securities [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Total marketable securities	25,869	14,943
Level 2 [Member] | Asset-backed Securities [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Total marketable securities	3,273	5,463
Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Total cash equivalents	0	0
Total marketable securities	0	0
Total	0	0
Level 3 [Member] | Money market funds [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Total cash equivalents	0	0
Level 3 [Member] | Commercial paper and corporate debt securities [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Total cash equivalents	0	0
Level 3 [Member] | U.S. Treasury securities and U.S. Government agency debt securities [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Total marketable securities	0	0
Level 3 [Member] | Certificates of deposits [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Total marketable securities	0	0
Level 3 [Member] | Commercial paper [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Total marketable securities	0	0
Level 3 [Member] | Municipal securities and corporate debt securities [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Total marketable securities	0	0
Level 3 [Member] | Asset-backed Securities [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Total marketable securities	$ 0	$ 0