Note 10 - Pension Plan and Other Postretirement Benefits - Benefit Payments, Which Reflect Expected Future Service, As Appropriate, Expected to Be Paid Out From Plan Assets (Details)	Dec. 31, 2017 USD ($)
Executive Survivor and Supplemental Retirement Plan [Member]
2018	$ 1,568,000
2019	1,612,000
2020	1,576,000
2021	1,670,000
2022	2,255,000
2023-2027	13,775,000
Pension Plan [Member]
2018	14,384
2019	15,022
2020	15,666
2021	16,336
2022	17,041
2023-2027	93,882
Other Postretirement Benefits Plan [Member]
2018	3,986,000
2019	4,107,000
2020	4,133,000
2021	4,218,000
2022	4,327,000
2023-2027	$ 21,089,000