Note 10 - Pension and Other Postretirement Benefit Plans (Details Textual) - USD ($)	3 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Directors' Plan [Member]
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year		$ 300,000
Defined Benefit Plan, Plan Assets, Contributions by Employer	$ 36,000
Retirement Plan [Member] | Savings Bank [Member]
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year	0
Other Postretirement Benefit Plans [Member]
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year		$ 300,000
Defined Benefit Plan, Plan Assets, Contributions by Employer	$ 15,000