Operations by Business Segment (Details) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2018 USD ($)	Jul. 01, 2017 USD ($)	Jun. 30, 2018 USD ($)	Jul. 01, 2017 USD ($)	Dec. 31, 2017 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments				2
Revenues		$ 270,649	$ 245,037	$ 479,300	$ 437,850
Income (loss) from operations		29,800	25,659	24,134	21,664
Interest expense		(1,754)	(1,072)	(3,155)	(2,329)
Interest income		101	73	179	143
Other income (expense), net		(1,052)	(1,339)	(2,714)	(2,478)
Income before income taxes		27,095	23,321	18,444	17,000
Segment assets, total		493,254	460,172	493,254	460,172	$ 473,135
Utility [Member]
Segment Reporting Information [Line Items]
Revenues		128,496	112,903	247,183	225,381
Income (loss) from operations		6,265	4,270	8,079	6,928
Segment assets, total		197,287	171,472	197,287	171,472
Residential commercial [Member]
Segment Reporting Information [Line Items]
Revenues		141,465	131,299	230,810	210,886
Income (loss) from operations		26,834	23,837	23,729	20,255
Segment assets, total		222,638	202,502	222,638	202,502
All other [Member]
Segment Reporting Information [Line Items]
Revenues		688	835	1,307	1,583
Income (loss) from operations		(2,764)	(965)	(6,556)	(3,193)
Segment assets, total		0	0	0	0
Reconciling adjustments [Member]
Segment Reporting Information [Line Items]
Revenues		0	0	0	0
Income (loss) from operations	[1]	(535)	(1,483)	(1,118)	(2,326)
Interest expense		(1,754)	(1,072)	(3,155)	(2,329)
Interest income		101	73	179	143
Other income (expense), net		(1,052)	(1,339)	(2,714)	(2,478)
Segment assets, total	[2]	$ 73,329	$ 86,198	$ 73,329	$ 86,198

[1] (a)Reclassification of depreciation expense and allocation of corporate expenses

[2] (b)Corporate assets include cash, prepaid expenses, corporate facilities, enterprise-wide information systems and other nonoperating assets.