Note 11 - Pension Plan and Other Postretirement Benefits - Components of Net Periodic Pension Benefit Cost (Details) (Parentheticals) - Executive Survivor and Supplemental Retirement Plan [Member] - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Amortization of Prior Service Costs from Other Comprehensive Income Charged to:	[1]	$ 10	$ 10	$ 19	$ 19
Amortization of Net Actuarial Loss from Other Comprehensive Income Charge to:	[2]	110	111	220	223
Electric Operation and Maintenance Expenses [Member]
Amortization of Prior Service Costs from Other Comprehensive Income Charged to:		4	4	8	8
Amortization of Net Actuarial Loss from Other Comprehensive Income Charge to:		66	68	132	136
Other Nonelectric Expenses [member]
Amortization of Prior Service Costs from Other Comprehensive Income Charged to:		6	6	11	11
Amortization of Net Actuarial Loss from Other Comprehensive Income Charge to:		$ 44	$ 43	$ 88	$ 87

[1] Amortization of Prior Service Costs from Other Comprehensive Income Charged to: Electric Operation and Maintenance Expenses $4 $4 $8 $8 Other Nonelectric Expenses 6 6 11 11

[2] Amortization of Net Actuarial Loss from Other Comprehensive Income Charged to: Electric Operation and Maintenance Expenses $66 $68 $132 $136 Other Nonelectric Expenses 44 43 88 87