INCOME TAXES. - Provision Reconciliation (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 26, 2015	Dec. 27, 2014
Reconciliation of the tax provision for income taxes (as a percent)
U.S. Federal statutory rate (as a percent)	35.00%	35.00%	35.00%
State income taxes, net of federal benefit (as a percent)	2.20%	2.40%	5.00%
Tax-exempt underwriting income of wholly-owned small captive insurance subsidiary (as a percent)	(1.40%)	(2.00%)	(2.40%)
Domestic production deduction (as a percent)	(3.00%)	(2.90%)	(2.30%)
Tax credits (as a percent)	(0.40%)	(0.50%)	(0.80%)
Other, net (as a percent)	0.70%	0.90%	(2.40%)
Effective tax rate (as a percent)	33.10%	32.90%	32.10%
Reconciliation of the provision for income taxes
U.S. Federal statutory rate	$ 9,955,459	$ 6,718,125	$ 4,364,717
State income taxes, net of federal benefit	623,253	470,203	617,410
Tax-exempt underwriting income of wholly-owned small captive insurance subsidiary	(413,339)	(388,168)	(297,243)
Domestic production deduction	(860,212)	(553,476)	(281,688)
Tax credits	(101,000)	(101,000)	(97,750)
Other, net	202,670	173,691	(304,791)
Effective tax rate	9,406,831	6,319,375	4,000,655
Reconciliation of the change in the unrecognized tax benefits
Unrecognized tax benefits at the beginning of the period	866,000	746,000	659,000
Gross increases - tax positions in prior periods	360,645	229,191	157,657
Lapse of statute of limitations	(136,645)	(109,191)	(70,657)
Unrecognized tax benefits at the end of the period	$ 1,090,000	$ 866,000	$ 746,000