Note 11 - Pension Plan and Other Postretirement Benefits - Amounts Recognized in Consolidated Balance Sheets (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension Plan [Member]
Regulatory Assets:
Unrecognized Prior Service Cost	$ 0	$ 5
Unrecognized Actuarial Loss	120,592	104,891
Total Regulatory Assets	120,592	104,896
Projected Benefit Obligation Liability – Net Amount Recognized	(384,785)	(328,442)	$ (352,718)
Unrecognized Prior Service Credit	0	5
Accumulated Other Comprehensive Loss:
Unrecognized Prior Service Cost	0	9
Unrecognized Actuarial (Gain) Loss	(82)	137
Total Accumulated Other Comprehensive Loss	(82)	146
Noncurrent Liability	55,004	58,659
Unrecognized Prior Service Cost	0	9
Executive Survivor and Supplemental Retirement Plan [Member]
Regulatory Assets:
Unrecognized Prior Service Cost	0	20
Unrecognized Actuarial Loss	2,170	1,768
Total Regulatory Assets	2,170	1,788
Projected Benefit Obligation Liability – Net Amount Recognized	(43,966)	(39,699)	(42,308)
Unrecognized Prior Service Credit	0	20
Accumulated Other Comprehensive Loss:
Unrecognized Prior Service Cost	1	64
Unrecognized Actuarial (Gain) Loss	9,170	6,455
Total Accumulated Other Comprehensive Loss	9,171	6,519
Unrecognized Prior Service Cost	1	64
Other Postretirement Benefits Plan [Member]
Regulatory Assets:
Unrecognized Prior Service Cost	(20,363)	0
Total Regulatory Assets	14,959	18,094
Projected Benefit Obligation Liability – Net Amount Recognized	(71,437)	(71,561)	$ (69,774)
Unrecognized Prior Service Credit	(20,363)	0
Unrecognized Net Actuarial Loss (Gain)	35,322	18,094
Accumulated Other Comprehensive Loss:
Unrecognized Prior Service Cost	(501)	0
Unrecognized Actuarial (Gain) Loss	184	(107)
Total Accumulated Other Comprehensive Loss	(317)	(107)
Unrecognized Prior Service Cost	$ (501)	$ 0