Commitments and contingent liabilities - Additional Information (Detail) - USD ($)		6 Months Ended
	Dec. 05, 2014	Jun. 28, 2015
Loss Contingencies [Line Items]
Estimated litigation liability		$ 2,600,000
Parish of Calcasieu | Judicial Ruling | Compensatory Damages
Loss Contingencies [Line Items]
Damages awarded	$ 100,000
Parish of Calcasieu | Judicial Ruling | Punitive Damages
Loss Contingencies [Line Items]
Damages awarded	$ 23,000,000
Parish of Calcasieu | Judicial Ruling | Unfavorable Regulatory Action
Loss Contingencies [Line Items]
Possible loss		10,000,000.0
Discontinued Operations
Loss Contingencies [Line Items]
Estimated litigation liability		$ 1,500,000
Minimum
Loss Contingencies [Line Items]
Time-frame over which the accrued amounts may be paid out, in years		15 years
Maximum
Loss Contingencies [Line Items]
Time-frame over which the accrued amounts may be paid out, in years		20 years
Accrued Liabilities
Loss Contingencies [Line Items]
Waste disposed accrued liability		$ 900,000
Other Liability
Loss Contingencies [Line Items]
Waste disposed accrued liability		$ 6,800,000