Operations by Business Segment (Details) $ in Thousands		3 Months Ended		6 Months Ended
		Jul. 01, 2017 USD ($)	Jul. 02, 2016 USD ($)	Jul. 01, 2017 USD ($)	Jul. 02, 2016 USD ($)	Dec. 31, 2016 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments				2
Revenues		$ 245,037	$ 224,763	$ 437,850	$ 405,596
Income (loss) from operations		25,312	24,286	20,970	19,166
Interest expense		(1,072)	(1,062)	(2,329)	(2,027)
Interest income		73	60	143	128
Other income (expense), net		(992)	(1,156)	(1,784)	(1,755)
Income before income taxes		23,321	22,128	17,000	15,512
Segment assets, total		460,172	426,144	460,172	426,144	$ 423,939
Utility services [Member]
Segment Reporting Information [Line Items]
Revenues		112,903	105,285	225,381	205,313
Income (loss) from operations		4,270	5,587	6,928	7,061
Segment assets, total		171,472	159,824	171,472	159,824
Residential commercial services [Member]
Segment Reporting Information [Line Items]
Revenues		131,299	118,945	210,886	199,279
Income (loss) from operations		23,837	21,226	20,255	19,790
Segment assets, total		202,502	181,945	202,502	181,945
All other [Member]
Segment Reporting Information [Line Items]
Revenues		835	533	1,583	1,004
Income (loss) from operations		(1,312)	(980)	(3,887)	(4,086)
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]	(1,483)	(1,547)	(2,326)	(3,599)
Interest expense		(1,072)	(1,062)	(2,329)	(2,027)
Interest income		73	60	143	128
Other income (expense), net		(992)	(1,156)	(1,784)	(1,755)
Segment assets, total	[2]	$ 86,198	$ 84,375	$ 86,198	$ 84,375

[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.