Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011 segment M Y town	Sep. 30, 2010	Sep. 30, 2011	Sep. 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	$ 164,284	$ 154,086	$ 444,105	$ 408,462	$ 576,919	$ 529,331
Depreciation and amortization	14,757	14,622	48,978	41,394	66,240	54,441
Federal and state income taxes	15,652	11,920	28,476	26,341	32,670	29,689
Operating income	36,450	31,873	77,431	62,230	95,696	78,366
Interest income	29	30	68	152	93	189
Interest expense	10,952	10,302	30,600	32,730	40,217	44,314
Income from AFUDC (debt and equity)	159	1,611	310	10,109	374	13,962
Net income	25,184	22,981	46,281	38,936	54,740	46,863
Capital Expenditures	24,093	22,200	76,460	78,826	106,354	115,006
Total assets	1,964,387		1,964,387		1,964,387		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	157,619	147,128	406,308	367,846	523,177	470,911
Depreciation and amortization	13,418	13,354	45,027	38,028	60,983	50,196
Federal and state income taxes	15,703	11,864	26,676	24,642	29,959	27,148
Operating income	35,359	30,827	71,430	56,700	87,258	70,541
Interest income	30	36	68	169	97	210
Interest expense	10,047	9,392	27,865	30,099	36,564	40,753
Income from AFUDC (debt and equity)	158	1,605	308	10,097	366	13,949
Net income	25,276	22,887	43,369	36,216	50,340	42,902
Capital Expenditures	20,800	21,081	70,685	74,862	96,532	109,992
Total assets	1,882,904		1,882,904		1,882,904		1,837,910
Gas
Statement of Operations Information, balance sheet information and capital expenditures of business segments
Revenues	5,052	5,403	33,344	36,480	47,750	52,993
Depreciation and amortization	873	844	2,617	2,184	3,465	2,691
Federal and state income taxes	(257)	(241)	1,111	981	1,751	1,619
Operating income	517	557	4,639	4,334	6,631	6,290
Interest income	75	80	204	339	269	400
Interest expense	979	989	2,933	2,959	3,915	3,946
Income from AFUDC (debt and equity)	1	6	2	12	8	13
Net income	(427)	(388)	1,792	1,553	2,840	2,462
Capital Expenditures	1,841	750	2,875	1,636	6,480	2,337
Total assets	141,679		141,679		141,679		139,532
Goodwill	39,492		39,492		39,492		39,492
Other.
Statement of Operations Information, balance sheet information and capital expenditures of business segments
Revenues	1,761	1,703	4,897	4,580	6,584	6,019
Depreciation and amortization	466	424	1,334	1,182	1,792	1,554
Federal and state income taxes	206	297	689	718	960	922
Operating income	574	489	1,362	1,196	1,807	1,535
Interest expense	2	7	6	28	11	36
Net income	335	482	1,120	1,167	1,560	1,499
Capital Expenditures	1,452	369	2,900	2,328	3,342	2,677
Total assets	25,658		25,658		25,658		23,163
Eliminations
Statement of Operations Information, balance sheet information and capital expenditures of business segments
Revenues	(148)	(148)	(444)	(444)	(592)	(592)
Interest income	(76)	(86)	(204)	(356)	(273)	(421)
Interest expense	(76)	(86)	(204)	(356)	(273)	(421)
Total assets	$ (85,854)		$ (85,854)		$ (85,854)		$ (79,294)