Debt: Line of Credit (Details) - Private Bank, Trust Company and BMO Harris Bank N A $ in Millions	12 Months Ended
	Dec. 30, 2017 USD ($) item	Jul. 31, 2017 USD ($)	Apr. 30, 2015 USD ($)
One month LIBOR
Line of Credit
Variable rate basis	one month LIBOR
Two month LIBOR
Line of Credit
Variable rate basis	two month LIBOR
Three month LIBOR
Line of Credit
Variable rate basis	three month LIBOR
Six month LIBOR
Line of Credit
Variable rate basis	six month LIBOR
Twelve month LIBOR
Line of Credit
Variable rate basis	twelve month LIBOR
Line of Credit
Line of Credit
Borrowings outstanding	$ 35.4
Terms notes, available borrowing to fund Tender Offer		$ 15.0	$ 30.0
Automatic deduction in the aggregate commitments under the line of credit effective July 2019 and each subsequent July thereafter through the term of the facility		$ 5.0
Line of credit available for additional borrowings	$ 14.6
Number of interest rate options | item	2
Non-utilization fees (as a percent)	0.25%
Line of Credit | Minimum
Line of Credit
Interest rate (as a percent)	3.42%
Line of Credit | Maximum
Line of Credit
Interest rate (as a percent)	4.50%
Line of Credit | Base Rate
Line of Credit
Variable rate basis	Base Rate
Applicable margin (as a percent)	0.00%
Line of Credit | LIBOR
Line of Credit
Variable rate basis	LIBOR
Applicable margin (as a percent)	2.00%