Commitments and contingencies - Additional Information (Detail) $ in Thousands, CAD in Millions	1 Months Ended		12 Months Ended
	Apr. 30, 2015 CAD	Jun. 22, 2012 CAD	Dec. 31, 2016 USD ($)	Dec. 26, 2015 USD ($)	Dec. 30, 2017 USD ($)
Commitments and Contingencies Disclosure [Line Items]
Standby letters of credit			$ 25,900		$ 32,300
Amounts drawn on letters of credit			0		0
Increase in estimated liability related to litigation				$ 2,800
Payments for legal settlements			17,400
Contingent liabilities related to legal matters			5,600		3,600
Bertico litigation
Commitments and Contingencies Disclosure [Line Items]
Litigation judgment | CAD	CAD 10.9	CAD 16.4
Performance guarantee
Commitments and Contingencies Disclosure [Line Items]
Accrued liabilities					1,200
Supply commitment
Commitments and Contingencies Disclosure [Line Items]
Guarantee obligation, maximum exposure			$ 136,200		$ 116,700