Employee benefit plans - Pension assets measured and recorded at fair value (Details) (Pension Plan [Member], USD $) In Thousands, unless otherwise specified	Dec. 26, 2014	Dec. 27, 2013	Dec. 28, 2012
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, fair value of plan assets	$ 35,028	$ 31,540	$ 35,117
Cash and Cash Equivalents [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, fair value of plan assets	100	100
Corporate Bonds [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, fair value of plan assets	30,900	27,500
Short-term Debt Securities [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, fair value of plan assets	600	500
Domestic Small-cap Value [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, fair value of plan assets	1,000	1,000
Domestic Mid-cap Value [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, fair value of plan assets	2,000	1,900
International Diversified Value [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, fair value of plan assets	400	500
Quoted Prices In Active markets for Identical Assets (Level 1) [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, fair value of plan assets	35,028	31,540
Quoted Prices In Active markets for Identical Assets (Level 1) [Member] | Cash and Cash Equivalents [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, fair value of plan assets	100	100
Quoted Prices In Active markets for Identical Assets (Level 1) [Member] | Corporate Bonds [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, fair value of plan assets	30,900	27,500
Quoted Prices In Active markets for Identical Assets (Level 1) [Member] | Short-term Debt Securities [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, fair value of plan assets	600	500
Quoted Prices In Active markets for Identical Assets (Level 1) [Member] | Domestic Small-cap Value [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, fair value of plan assets	1,000	1,000
Quoted Prices In Active markets for Identical Assets (Level 1) [Member] | Domestic Mid-cap Value [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, fair value of plan assets	2,000	1,900
Quoted Prices In Active markets for Identical Assets (Level 1) [Member] | International Diversified Value [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, fair value of plan assets	400	500
Significant Other Observable Inputs (Level 2) [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, fair value of plan assets	0	0
Significant Other Observable Inputs (Level 2) [Member] | Cash and Cash Equivalents [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, fair value of plan assets	0	0
Significant Other Observable Inputs (Level 2) [Member] | Corporate Bonds [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, fair value of plan assets	0	0
Significant Other Observable Inputs (Level 2) [Member] | Short-term Debt Securities [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, fair value of plan assets	0	0
Significant Other Observable Inputs (Level 2) [Member] | Domestic Small-cap Value [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, fair value of plan assets	0	0
Significant Other Observable Inputs (Level 2) [Member] | Domestic Mid-cap Value [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, fair value of plan assets	0	0
Significant Other Observable Inputs (Level 2) [Member] | International Diversified Value [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, fair value of plan assets	0	0
Significant Unobservable Inputs (Level 3) [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, fair value of plan assets	0	0
Significant Unobservable Inputs (Level 3) [Member] | Cash and Cash Equivalents [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, fair value of plan assets	0	0
Significant Unobservable Inputs (Level 3) [Member] | Corporate Bonds [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, fair value of plan assets	0	0
Significant Unobservable Inputs (Level 3) [Member] | Short-term Debt Securities [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, fair value of plan assets	0	0
Significant Unobservable Inputs (Level 3) [Member] | Domestic Small-cap Value [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, fair value of plan assets	0	0
Significant Unobservable Inputs (Level 3) [Member] | Domestic Mid-cap Value [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, fair value of plan assets	0	0
Significant Unobservable Inputs (Level 3) [Member] | International Diversified Value [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, fair value of plan assets	$ 0	$ 0