Note 10 - Pension and Other Postretirement Benefit Plans - Components of Net Expense for Pension, Outside, and Other Postretirement Benefit Plans (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Employee Pension Plan [Member]
Net employee pension benefit	$ (7,000)	$ (12,000)	$ (13,000)	$ (25,000)
Employee Pension Plan [Member]
Interest cost	199,000	195,000	398,000	390,000
Amortization of unrecognized loss	66,000	156,000	133,000	311,000
Expected return on plan assets	(272,000)	(363,000)	(544,000)	(726,000)
Directors' Plan [Member]
Interest cost	21,000	20,000	42,000	40,000
Amortization of unrecognized loss	(35,000)	(23,000)	(70,000)	(46,000)
Net employee pension benefit	(4,000)	11,000	(8,000)	22,000
Service cost	10,000	11,000	20,000	22,000
Amortization of past service liability	0	3,000	0	6,000
Other Postretirement Benefit Plans [Member]
Interest cost	85,000	77,000	170,000	154,000
Net employee pension benefit	135,000	153,000	268,000	305,000
Service cost	70,000	88,000	140,000	176,000
Amortization of past service liability	$ (20,000)	$ (12,000)	$ (42,000)	$ (25,000)