Divestiture-related activities - Schedule of Operating Results of Operations Treated as Discontinued Operations (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Dec. 31, 2016	Sep. 25, 2016	Jun. 26, 2016	Mar. 27, 2016	Dec. 31, 2015	Sep. 27, 2015	Jun. 28, 2015	Mar. 29, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Divestiture-Related Activities [Abstract]
Costs and other expenses									$ 922	$ 1,730	$ 3,407
Loss from discontinued operations before income taxes									(922)	(1,730)	(3,407)
Tax benefit on loss from discontinued operations									1,112	10,635	698
Income (loss) on discontinued operations	$ 187	$ 122	$ 193	$ (312)	$ 10,517	$ (719)	$ (190)	$ (703)	$ 190	$ 8,905	$ (2,709)

Amount before tax of income (loss) from a discontinued operation. Includes, but is not limited to, the income (loss) from operations during the phase-out period, gain (loss) on disposal, gain (loss) for reversal of write-down (write-down) to fair value, less cost to sell, and adjustments to a prior period gain (loss) on disposal.

Amount of tax expense (benefit) related to a discontinued operation. Includes, but is not limited to, tax expense (benefit) related to income (loss) from operations during the phase-out period, tax expense (benefit) related to gain (loss) on disposal, tax expense (benefit) related to gain (loss) for reversal of write-down (write-down) to fair value, less cost to sell, and tax expense (benefit) related to adjustments of a prior period gain (loss) on disposal.

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- Definition

Amount after tax of income (loss) from a discontinued operation including the portion attributable to the noncontrolling interest. Includes, but is not limited to, the income (loss) from operations during the phase-out period, gain (loss) on disposal, gain (loss) for reversal of write-down (write-down) to fair value, less cost to sell, and adjustments to a prior period gain (loss) on disposal.

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