Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012 segment town	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011
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	$ 131,632	$ 129,093	$ 268,776	$ 279,821	$ 565,825	$ 566,721
Depreciation and amortization	15,068	16,888	30,003	34,221	59,318	66,106
Federal and state income taxes	6,760	5,579	12,959	12,824	34,433	28,938
Operating income	20,762	19,134	41,573	40,981	97,525	91,119
Interest income	123	16	302	40	819	94
Interest expense	10,059	9,938	21,002	19,648	42,873	39,567
Income from AFUDC (debt and equity)	171	128	270	151	630	1,826
Net Income	10,708	9,175	20,512	21,097	54,387	52,537
Capital Expenditures	35,183	34,809	69,970	52,367	118,781	103,795
Total assets	2,049,552		2,049,552		2,049,552		2,021,835
Goodwill	39,492		39,492		39,492		39,492
Electric
Statement of Income Information, balance sheet information and capital expenditures of business segments
Revenues	124,091	120,329	243,817	248,690	519,403	512,686
Depreciation and amortization	13,759	15,582	27,329	31,610	53,956	60,919
Federal and state income taxes	6,745	5,340	11,932	10,973	32,602	26,120
Operating income	19,834	17,795	38,078	36,071	90,596	82,725
Interest income	118	16	288	38	805	103
Interest expense	9,174	9,017	19,202	17,818	39,243	35,908
Income from AFUDC (debt and equity)	170	128	268	151	626	1,814
Net Income	10,691	8,792	18,864	18,093	51,441	47,951
Capital Expenditures	33,745	33,034	66,863	49,885	110,478	96,148
Total assets	1,966,579		1,966,579		1,966,579		1,931,320
Gas
Statement of Income Information, balance sheet information and capital expenditures of business segments
Revenues	5,804	7,303	21,487	28,292	39,625	48,101
Depreciation and amortization	861	870	1,780	1,743	3,530	3,436
Federal and state income taxes	(238)	(13)	459	1,368	766	1,767
Operating income	534	928	2,588	4,122	4,980	6,672
Interest income	95	58	166	130	296	273
Interest expense	976	978	1,953	1,954	3,909	3,926
Income from AFUDC (debt and equity)	1		2		4	12
Net Income	(394)	(27)	725	2,219	1,215	2,879
Capital Expenditures	844	694	1,569	1,034	4,657	5,389
Total assets	144,958		144,958		144,958		145,897
Goodwill	39,492		39,492		39,492		39,492
Other
Statement of Income Information, balance sheet information and capital expenditures of business segments
Revenues	1,885	1,609	3,768	3,135	7,389	6,526
Depreciation and amortization	448	436	894	868	1,832	1,751
Federal and state income taxes	253	252	568	483	1,065	1,051
Operating income	394	411	907	788	1,949	1,722
Interest income	1		1		1
Interest expense		1		4	4	15
Net Income	411	410	923	785	1,731	1,707
Capital Expenditures	594	1,081	1,538	1,448	3,646	2,258
Total assets	27,083		27,083		27,083		26,038
Eliminations
Statement of Income Information, balance sheet information and capital expenditures of business segments
Revenues	(148)	(148)	(296)	(296)	(592)	(592)
Interest income	(91)	(58)	(153)	(128)	(283)	(282)
Interest expense	(91)	(58)	(153)	(128)	(283)	(282)
Total assets	$ (89,068)		$ (89,068)		$ (89,068)		$ (81,420)