Note 7 - Employee Benefits Plans - Assumptions Used to Determine the Plan's Financial Status and Postretirement Benefit Cost (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Deferred Compensation Plan [Member]
Discount rate for funded status	3.15%	3.50%
Discount rate for benefit cost	3.50%	3.55%
Supplemental Employee Retirement Plan [Member]
Discount rate for funded status	3.20%	3.55%
Discount rate for benefit cost	3.55%	3.65%
Health care cost assumed trend rate for next year
Rate that the cost trend rate gradually declines to
Year that the rate reaches the rate it is assumed to remain at
Other Postretirement Benefits Plan [Member]
Discount rate for funded status	2.95%	3.20%
Discount rate for benefit cost	3.20%	3.20%
Health care cost assumed trend rate for next year		6.00%
Rate that the cost trend rate gradually declines to		5.50%
Year that the rate reaches the rate it is assumed to remain at		2018