Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010
Segment Information
Number of business segments			3
Number of towns to which water service is provided			3
Statement of Operations Information, balance sheet information and capital expenditures of business segments
Revenues	$ 129,093	$ 114,482	$ 279,821	$ 254,376	$ 566,721	$ 503,299
Depreciation and amortization	16,888	13,586	34,221	26,771	66,106	52,853
Federal and state income taxes	5,579	4,485	12,824	14,419	28,938	24,646
Operating income	19,134	14,279	40,981	30,357	91,119	70,170
Interest income	16	52	40	122	94	202
Interest expense	9,938	11,003	19,648	22,428	39,567	46,047
Income from AFUDC (debt and equity)	128	4,343	151	8,498	1,826	15,691
Net income	9,175	7,369	21,097	15,955	52,537	38,711
Capital Expenditures	34,809	25,751	52,367	56,626	103,795	131,306
Total assets	1,930,983		1,930,983		1,930,983		1,921,311
Goodwill	39,492		39,492		39,492		39,492
Electric
Statement of Operations Information, balance sheet information and capital expenditures of business segments
Revenues	120,329	106,686	248,690	220,718	512,686	445,745
Depreciation and amortization	15,582	12,373	31,610	24,674	60,919	48,992
Federal and state income taxes	5,340	4,497	10,973	12,778	26,120	22,598
Operating income	17,795	13,388	36,071	25,873	82,725	63,089
Interest income	16	61	38	133	103	222
Interest expense	9,017	10,147	17,818	20,707	35,908	42,481
Income from AFUDC (debt and equity)	128	4,337	151	8,492	1,814	15,684
Net income	8,792	7,405	18,093	13,329	47,951	35,546
Capital Expenditures	33,034	24,545	49,885	53,781	96,148	126,589
Total assets	1,854,569		1,854,569		1,854,569		1,837,910
Gas
Statement of Operations Information, balance sheet information and capital expenditures of business segments
Revenues	7,303	6,516	28,292	31,077	48,101	52,384
Depreciation and amortization	870	831	1,743	1,339	3,436	2,352
Federal and state income taxes	(13)	(219)	1,368	1,220	1,767	1,216
Operating income	928	543	4,122	3,778	6,672	5,643
Interest income	58	133	130	259	273	396
Interest expense	978	986	1,954	1,969	3,926	3,943
Income from AFUDC (debt and equity)		6		6	12	7
Net income	(27)	(372)	2,219	1,942	2,879	1,813
Capital Expenditures	694	598	1,034	886	5,389	2,287
Total assets	140,712		140,712		140,712		139,532
Other.
Statement of Operations Information, balance sheet information and capital expenditures of business segments
Revenues	1,609	1,428	3,135	2,877	6,526	5,762
Depreciation and amortization	436	382	868	758	1,751	1,509
Federal and state income taxes	252	207	483	421	1,051	832
Operating income	411	348	788	706	1,722	1,438
Interest expense	1	12	4	22	15	39
Net income	410	336	785	684	1,707	1,352
Capital Expenditures	1,081	608	1,448	1,959	2,258	2,430
Total assets	24,349		24,349		24,349		23,163
Eliminations
Statement of Operations Information, balance sheet information and capital expenditures of business segments
Revenues	(148)	(148)	(296)	(296)	(592)	(592)
Interest income	(58)	(142)	(128)	(270)	(282)	(416)
Interest expense	(58)	(142)	(128)	(270)	(282)	(416)
Total assets	$ (88,647)		$ (88,647)		$ (88,647)		$ (79,294)