Business Combinations and Segment Information - Information on continuing operations for business segments (Details 1) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Segment Reporting Information [Line Items]
Operating Revenue	$ 197,175	$ 200,023	$ 606,899	$ 591,017
Interest Charges	8,026	7,730	23,996	23,175
Income Tax Expense - Continuing Operations	5,163	6,521	15,738	14,602
Net Income (Loss)	14,616	15,392	44,811	44,763
Intersegment Eliminations
Segment Reporting Information [Line Items]
Operating Revenue	(11)	(29)	(27)	(84)
Corporate and Intersegment Eliminations
Segment Reporting Information [Line Items]
Interest Charges	475	504	1,484	1,542
Corporate
Segment Reporting Information [Line Items]
Income Tax Expense - Continuing Operations	(1,326)	(1,301)	(3,722)	(4,068)
Net Income (Loss)	(1,511)	(2,460)	(3,650)	(5,933)
Discontinued Operations
Segment Reporting Information [Line Items]
Net Income (Loss)	22	(317)	171	1,616
Electric | Operating Segments
Segment Reporting Information [Line Items]
Operating Revenue	102,723	100,567	313,642	305,078
Interest Charges	6,304	6,069	18,744	18,273
Income Tax Expense - Continuing Operations	4,730	4,761	11,262	9,995
Net Income (Loss)	12,513	12,921	34,199	34,351
Manufacturing | Operating Segments
Segment Reporting Information [Line Items]
Operating Revenue	52,171	52,460	170,443	160,492
Interest Charges	974	900	2,972	2,578
Income Tax Expense - Continuing Operations	182	855	2,992	2,516
Net Income (Loss)	1,246	1,714	6,108	4,810
Plastics | Operating Segments
Segment Reporting Information [Line Items]
Operating Revenue	42,292	47,025	122,841	125,531
Interest Charges	273	257	796	782
Income Tax Expense - Continuing Operations	1,577	2,206	5,206	6,159
Net Income (Loss)	$ 2,346	$ 3,534	$ 7,983	$ 9,919