Note 9 - Pension and Other Postretirement Benefit Plans (Details Textual) - USD ($)	6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Directors' Plan [Member]
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year		$ 200,000
Defined Benefit Plan, Plan Assets, Contributions by Employer	$ 48,000
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		$ 200,000
Defined Benefit Plan, Plan Assets, Contributions by Employer	$ 56,000