Goodwill and intangible assets (Schedule of intangible assets amortization expense) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jul. 04, 2015	Jun. 28, 2014	Jul. 04, 2015	Jun. 28, 2014
Goodwill and Intangible Assets Disclosure [Abstract]
Amortization expense	$ 6,264	$ 6,509	$ 11,370	$ 12,547
Estimated amortization expense for the years ended:
2015	28,321		28,321
2016	34,864		34,864
2017	33,449		33,449
2018	24,757		24,757
2019	$ 21,711		$ 21,711