Note 8 - Pension and Other Postretirement Benefit Plans (Details Textual) - USD ($)	9 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Directors' Plan [Member]
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year		$ 300,000
Defined Benefit Plan, Plan Assets, Contributions by Employer	$ 108,000
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	$ 60,000