Note 2 - Allowance for Doubtful Accounts Receivable - Allowance for Doubtful Accounts Receivable (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Balance at the beginning of year	$ 848,000	$ 680,000	$ 560,000
Provision for bad debts - accounts receivable	512,000	266,000	197,000
Charge-offs	(96,000)	(98,000)	(105,000)
Recoveries	1,000	0	28,000
Balance at the end of year	$ 1,276,000	$ 848,000	$ 680,000