Note 11 - Pension Plan and Other Postretirement Benefits - Components of Net Periodic Pension Benefit Cost (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Pension Plan [Member]
Amortization of Net Actuarial Loss:
From Regulatory Asset		$ 1,272	$ 1,227	$ 2,545	$ 2,454
From Other Comprehensive Income	[1]	32	32	63	63
Net Periodic Pension Cost		1,470	1,343	2,939	2,686
Service Cost—Benefit Earned During the Period		1,407	1,381	2,814	2,763
Interest Cost on Projected Benefit Obligation		3,536	3,521	7,070	7,043
Expected Return on Assets		(4,807)	(4,866)	(9,614)	(9,733)
Amortization of Prior-Service Cost:
From Regulatory Asset		29	47	59	94
From Other Comprehensive Income	[1]	1	1	2	2
Executive Survivor and Supplemental Retirement Plan [Member]
Amortization of Net Actuarial Loss:
From Regulatory Asset		72	73	143	146
From Other Comprehensive Income	[2]	110	111	220	223
Net Periodic Pension Cost		689	678	1,378	1,356
Service Cost—Benefit Earned During the Period		72	63	145	126
Interest Cost on Projected Benefit Obligation		421	417	843	834
Amortization of Prior-Service Cost:
From Regulatory Asset		4	4	8	8
From Other Comprehensive Income	[3]	10	10	19	19
Postretirement Benefits [Member]
Amortization of Net Actuarial Loss:
From Regulatory Asset		233		466
From Other Comprehensive Income	[1]	6		12
Net Periodic Pension Cost		1,273	881	2,546	1,762
Service Cost—Benefit Earned During the Period		356	305	712	611
Interest Cost on Projected Benefit Obligation		678	542	1,356	1,083
Amortization of Prior-Service Cost:
From Regulatory Asset			33		66
From Other Comprehensive Income	[1]		1		2
Effect of Medicare Part D Subsidy		$ (140)	$ (258)	$ (280)	$ (515)

[1] Corporate cost included in Other Nonelectric Expenses.

[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

[3] 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