Components of accumulated other comprehensive loss (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 26, 2014		Mar. 27, 2013
Components of Change in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated other comprehensive loss, net of tax, beginning balance	$ (16,842)
Amortization of net loss, before tax	231	[1]
Net change in fair value of derivatives, before tax	(573)
Unrealized gain on hedged transactions, tax benefit	224		0
Other comprehensive loss, tax benefit	134
Accumulated other comprehensive loss, net of tax, ending balance	(17,050)
Pensions [Member]
Components of Change in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated other comprehensive loss, net of tax, beginning balance	(18,690)
Amortization of net loss, before tax	231	[1]
Minimum pension liability, adjustment, tax expense	(90)
Accumulated other comprehensive loss, net of tax, ending balance	(18,549)
Derivatives [Member]
Components of Change in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated other comprehensive loss, net of tax, beginning balance	1,848
Net change in fair value of derivatives, before tax	(573)
Unrealized gain on hedged transactions, tax benefit	224
Accumulated other comprehensive loss, net of tax, ending balance	$ 1,499

[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 quarter ended March 26, 2014. See Note 8 for additional details.