Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013 segment town	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Segment Information
Number of business segments			3
Number of towns to which water service is provided			3
Statement of Income Information, balance sheet information and capital expenditures of business segments
Revenues	$ 136,646	$ 131,632	$ 287,786	$ 268,776	$ 576,107	$ 565,825
Depreciation and amortization	17,635	15,068	33,736	30,003	64,180	59,318
Federal and state income taxes	7,049	6,760	14,531	12,959	35,730	34,433
Operating income	21,110	20,762	42,968	41,573	97,615	97,525
Interest income	10	123	517	302	1,187	819
Interest expense	10,504	10,059	20,754	21,002	41,220	42,873
Income from AFUDC (debt and equity)	1,339	171	2,170	270	3,829	630
Net Income	11,658	10,708	24,287	20,512	59,456	54,387
Capital Expenditures	38,500	35,183	76,208	69,970	155,820	118,781
Total assets	2,168,637		2,168,637		2,168,637		2,126,369
Goodwill	39,492		39,492		39,492		39,492
Electric
Statement of Income Information, balance sheet information and capital expenditures of business segments
Revenues	127,026	124,091	255,788	243,817	522,624	519,403
Depreciation and amortization	16,205	13,759	30,887	27,329	58,869	53,956
Federal and state income taxes	6,948	6,745	12,943	11,932	33,277	32,602
Operating income	19,994	19,834	38,509	38,078	89,876	90,596
Interest income	3	118	497	288	1,155	805
Interest expense	9,557	9,174	18,893	19,202	37,558	39,243
Income from AFUDC (debt and equity)	1,331	170	2,161	268	3,811	626
Net Income	11,498	10,691	21,721	18,864	55,487	51,441
Capital Expenditures	36,535	33,745	73,070	66,863	149,619	110,478
Total assets	2,082,513		2,082,513		2,082,513		2,034,399
Gas
Statement of Income Information, balance sheet information and capital expenditures of business segments
Revenues	7,777	5,804	28,270	21,487	46,632	39,625
Depreciation and amortization	927	861	1,851	1,780	3,669	3,530
Federal and state income taxes	(129)	(238)	1,076	459	1,406	766
Operating income	744	534	3,639	2,588	6,055	4,980
Interest income	34	95	105	166	262	296
Interest expense	976	976	1,953	1,953	3,905	3,909
Income from AFUDC (debt and equity)	8	1	9	2	18	4
Net Income	(214)	(394)	1,735	725	2,267	1,215
Capital Expenditures	1,463	844	2,196	1,569	4,198	4,657
Total assets	150,733		150,733		150,733		148,814
Other
Statement of Income Information, balance sheet information and capital expenditures of business segments
Revenues	1,991	1,885	4,024	3,768	7,443	7,389
Depreciation and amortization	503	448	998	894	1,642	1,832
Federal and state income taxes	230	253	512	568	1,047	1,065
Operating income	372	394	820	907	1,684	1,949
Interest income	2	1	7	1	13	1
Interest expense						4
Net Income	374	411	831	923	1,702	1,731
Capital Expenditures	502	594	942	1,538	2,003	3,646
Total assets	29,494		29,494		29,494		28,871
Eliminations
Statement of Income Information, balance sheet information and capital expenditures of business segments
Revenues	(148)	(148)	(296)	(296)	(592)	(592)
Interest income	(29)	(91)	(92)	(153)	(243)	(283)
Interest expense	(29)	(91)	(92)	(153)	(243)	(283)
Total assets	$ (94,103)		$ (94,103)		$ (94,103)		$ (85,715)