Operating Leases: (Details)	6 Months Ended
	Jun. 29, 2019 USD ($) item	Jun. 30, 2018 USD ($)
Operating Leases
Weighted average remaining lease term	10 years 3 months 18 days
Weighted average discount rate (as a percent)	5.50%
Rent expense	$ 720,600
Rent expense		$ 574,300
Maturities of operating lease liabilities:
2019	140,200
2020	762,500
2021	783,600
2022	784,400
2023	763,300
Thereafter	5,042,900
Total lease payments	8,276,900
Less imputed interest	(2,013,200)
Present value of lease liabilities	6,263,700
Operating lease liability
Operating lease liability, current	$ 200,000
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List]	Accrued Liabilities, Current
Cash paid for amounts included in the measurement of lease liabilities:
Operating cash flow outflow from operating leases	$ 353,700
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