Litigation and contingencies Litigation and Contingencies (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 27, 2012
Commitments and Contingencies Disclosure [Abstract]
Loss Contingency, Range of Possible Loss, Maximum	$ 2.8
Litigation for Violation of Americans with Disabilities Act
Loss Contingencies [Line Items]
Capital Expenditures Incurred but Not yet Paid	$ 0.5