Fair Value Measurements and Disclosures, and Financial Instruments - Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 26, 2015	Dec. 27, 2014
Assets:
Total marketable securities	$ 84,626	$ 83,962
Fair Value, Measurements, Recurring [Member]
Assets:
Total cash equivalents	7,076	610
Total marketable securities	43,065	49,286
Total	50,141	49,896
Liabilities:
Contingent consideration payable	1,920	2,397
Fair Value, Measurements, Recurring [Member] | Money market funds [Member]
Assets:
Total cash equivalents	642	610
Fair Value, Measurements, Recurring [Member] | Commercial paper [Member]
Assets:
Total cash equivalents	6,434	0
Fair Value, Measurements, Recurring [Member] | U.S. Treasury, U.S. Government and U.S. Government agency debt securities [Member]
Assets:
Total marketable securities	21,684	23,034
Fair Value, Measurements, Recurring [Member] | Commercial paper, municipal securities and corporate debt securities [Member]
Assets:
Total marketable securities	21,381	26,252
Fair Value, Measurements, Recurring [Member] | Cash [Member]
Assets:
Total cash equivalents	34,500	34,100
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Assets:
Total cash equivalents	642	610
Total marketable securities	0	2,497
Total	642	3,107
Liabilities:
Contingent consideration payable	0	0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Money market funds [Member]
Assets:
Total cash equivalents	642	610
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Commercial paper [Member]
Assets:
Total cash equivalents	0	0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | U.S. Treasury, U.S. Government and U.S. Government agency debt securities [Member]
Assets:
Total marketable securities	0	2,497
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Commercial paper, municipal securities and corporate debt securities [Member]
Assets:
Total marketable securities	0	0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Assets:
Total cash equivalents	6,434	0
Total marketable securities	43,065	46,789
Total	49,499	46,789
Liabilities:
Contingent consideration payable	0	0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Money market funds [Member]
Assets:
Total cash equivalents	0	0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Commercial paper [Member]
Assets:
Total cash equivalents	6,434	0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | U.S. Treasury, U.S. Government and U.S. Government agency debt securities [Member]
Assets:
Total marketable securities	21,684	20,537
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Commercial paper, municipal securities and corporate debt securities [Member]
Assets:
Total marketable securities	21,381	26,252
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Assets:
Total cash equivalents	0	0
Total marketable securities	0	0
Total	0	0
Liabilities:
Contingent consideration payable	1,920	2,397
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Money market funds [Member]
Assets:
Total cash equivalents	0	0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Commercial paper [Member]
Assets:
Total cash equivalents	0	0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | U.S. Treasury, U.S. Government and U.S. Government agency debt securities [Member]
Assets:
Total marketable securities	0	0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Commercial paper, municipal securities and corporate debt securities [Member]
Assets:
Total marketable securities	$ 0	$ 0