Summary of significant accounting policies - Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 30, 2017	Dec. 31, 2016
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Assets	$ 10,836	$ 9,271
Liabilities	13,543	11,126
Company-owned Life Insurance [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Assets	10,836	9,271
Deferred compensation liabilities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Liabilities	13,543	11,126
Significant other observable inputs (Level 2)
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Assets	10,836	9,271
Liabilities	13,543	11,126
Significant other observable inputs (Level 2) | Company-owned Life Insurance [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Assets	10,836	9,271
Significant other observable inputs (Level 2) | Deferred compensation liabilities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Liabilities	$ 13,543	$ 11,126