Note 2 - Segment Information - Information on Continuing Operations for Business Segments (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Annual Revenue	$ 212,086	$ 203,482	$ 426,203	$ 409,724
Interest Charges	7,527	7,976	14,989	15,970
Income Tax Expense—Continuing Operations	5,897	5,083	12,260	10,575
Net Income	16,778	15,675	36,363	30,195
Assets	1,944,056		1,944,056		$ 1,912,385
Discontinued Operations [Member]
Net Income	61	119	117	149
Operating Segments [Member] | Electric [Member]
Annual Revenue	102,236	97,925	220,787	210,919
Interest Charges	6,439	6,156	12,825	12,440
Income Tax Expense—Continuing Operations	2,442	1,920	8,504	6,532
Net Income	10,134	9,148	25,694	21,686
Assets	1,639,699		1,639,699		1,622,231
Operating Segments [Member] | Manufacturing [Member]
Annual Revenue	59,304	58,452	117,721	118,272
Interest Charges	553	1,006	1,107	1,998
Income Tax Expense—Continuing Operations	1,573	1,791	2,628	2,810
Net Income	2,955	3,009	5,127	4,862
Assets	170,429		170,429		166,525
Operating Segments [Member] | Plastics [Member]
Annual Revenue	50,551	47,112	87,708	80,549
Interest Charges	173	279	326	523
Income Tax Expense—Continuing Operations	2,858	2,262	4,248	3,629
Net Income	4,637	3,485	7,074	5,637
Assets	94,900		94,900		84,592
Intersegment Eliminations [Member]
Annual Revenue	(5)	(7)	(13)	(16)
Corporate and Eliminations [Member]
Interest Charges	362	535	731	1,009
Corporate, Non-Segment [Member]
Income Tax Expense—Continuing Operations	(976)	(890)	(3,120)	(2,396)
Net Income	(1,009)	$ (86)	(1,649)	$ (2,139)
Assets	$ 39,028		$ 39,028		$ 39,037