Cash, cash equivalents and short-term investments (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 28, 2012	Dec. 30, 2011
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 134,109	$ 109,612
Unrealized Gains	41	91
Unrealized Losses	(1)	(4)
Estimated Fair Value	134,149	109,699
Cash
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	64,134	39,991
Unrealized Gains	0	0
Unrealized Losses	0	0
Estimated Fair Value	64,134	39,991
Money market securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	49,134	44,448
Unrealized Gains	0	0
Unrealized Losses	0	0
Estimated Fair Value	49,134	44,448
Total cash equivalents
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	49,134	44,448
Unrealized Gains	0	0
Unrealized Losses	0	0
Estimated Fair Value	49,134	44,448
Total cash and cash equivalents
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	113,268	84,439
Unrealized Gains	0	0
Unrealized Losses	0	0
Estimated Fair Value	113,268	84,439
State and municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	20,841	25,173
Unrealized Gains	41	91
Unrealized Losses	(1)	(4)
Estimated Fair Value	20,881	25,260
Total short-term investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	20,841	25,173
Unrealized Gains	41	91
Unrealized Losses	(1)	(4)
Estimated Fair Value	$ 20,881	$ 25,260