Operating Leases: (Details)	12 Months Ended
	Dec. 28, 2019 USD ($) item	Dec. 29, 2018 USD ($)	Dec. 30, 2017 USD ($)
Operating Leases
Weighted average remaining lease term	9 years 10 months 24 days
Weighted average discount rate (as a percent)	5.50%
Rent expense	$ 1,358,000
Rent expense		$ 1,235,000	$ 1,102,000
Corporate headquarters, Minnesota
Operating Leases
Lease renewal option	true
Number of lease extension periods | item	2
Lease renewal term	5 years
Satellite office space, California
Operating Leases
Lease renewal option	true
Lease renewal term	3 years