Goodwill and intangible assets (Major Components - Intangible Assets) (Details) - USD ($) $ in Thousands	Dec. 30, 2017	Dec. 31, 2016	Jan. 02, 2016
Finite-Lived Intangible Assets [Line Items]
Cost	$ 213,095	$ 214,513
Accumulated amortization	41,036	30,989
Net book value	172,059	183,524	$ 195,008
Customer Relationships [Member]
Finite-Lived Intangible Assets [Line Items]
Cost	211,176	212,172
Accumulated amortization	39,274	28,914
Net book value	171,902	183,258	194,633
Patents, Trademarks and Other
Finite-Lived Intangible Assets [Line Items]
Cost	1,919	2,341
Accumulated amortization	1,762	2,075
Net book value	$ 157	$ 266	$ 375