Components of accumulated other comprehensive loss (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 24, 2014
Components of Change in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance as of December 25, 2013	$ (16,842)
Amortization of net loss, before tax	693	[1]
Net change in fair value of derivatives, before tax	(1,178)
Income tax (expense) benefit related to items of other comprehensive income	189
Balance as of September 24, 2014	(17,138)
Pensions [Member]
Components of Change in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance as of December 25, 2013	(18,690)
Amortization of net loss, before tax	693	[1]
Income tax (expense) benefit related to items of other comprehensive income	(271)
Balance as of September 24, 2014	(18,268)
Derivatives [Member]
Components of Change in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance as of December 25, 2013	1,848
Net change in fair value of derivatives, before tax	(1,178)
Income tax (expense) benefit related to items of other comprehensive income	460
Balance as of September 24, 2014	$ 1,130

[1] Before-tax amount that was reclassified from accumulated other comprehensive loss and included as a component of pension expense within general and administrative expenses in our Condensed Consolidated Statements of Income during the three quarters ended September 24, 2014. See Note 8 for additional details.