Shareholders' Equity: (Details) (USD $)	12 Months Ended
	Dec. 27, 2014	Dec. 28, 2013	Dec. 29, 2012
Repurchase of Common Stock
Aggregate purchase price of shares repurchased (in dollars)	$ 11,564,800	$ 1,854,900	$ 7,220,300
Dividends
Cash dividend approved by Board of Directors (in dollars per share)	$ 0.23	$ 0.19	$ 0.15
Special cash dividend approved by Board of Directors (in dollars per share)	$ 5.00		$ 5.00
Total special dividend in cash	25,800,000		25,400,000
Borrowings on line of credit for special dividend	13,000,000		12,800,000
Aggregate quarterly cash dividends declared and paid	1,200,000	960,000	750,000
Common Stock Repurchase Program
Repurchase of Common Stock
Number of shares repurchased	166,030	28,422	134,720
Aggregate purchase price of shares repurchased (in dollars)	$ 11,600,000	$ 1,900,000	$ 7,200,000
Aggregate purchase price per share of shares repurchased (in dollars per share)	$ 69.65	$ 65.26	$ 53.60
Number of additional shares that can be repurchased	166,700