Employee benefit plans - Financial status of pension plan (Details) (Pension Plan [Member], USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 26, 2014	Dec. 27, 2013
Pension Plan [Member]
Change in benefit obligation:
Projected benefit obligation at beginning of year	$ 36,919	$ 40,636
Service cost	0	19
Interest cost	1,713	1,616
Actuarial losses (gains)	5,107	(3,280)
Benefits paid	(1,998)	(2,056)
Plan curtailments	0	(16)
Projected benefit obligation at end of year	41,741	36,919
Change in fair value of plan assets:
Fair value of plan assets at beginning of year	31,540	35,117
Actual return on plan assets	5,394	(1,644)
Employer contributions	92	123
Benefits paid	(1,998)	(2,056)
Fair value of plan assets at end of year	35,028	31,540
Amounts recognized on the Consolidated Balance Sheets:
Non-current assets (included in deferred loan costs and other assets)	290	263
Non-current liabilities (included in other-long term liabilities)	(7,003)	(5,642)
Net amount recorded	$ (6,713)	$ (5,379)