Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012 segment town	Sep. 30, 2011	Sep. 30, 2012	Sep. 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	$ 159,202	$ 164,284	$ 427,977	$ 444,105	$ 560,743	$ 576,919
Depreciation and amortization	15,108	14,757	45,111	48,978	59,669	66,240
Federal and state income taxes	15,477	15,652	28,436	28,476	34,258	32,670
Operating income	35,282	36,450	76,854	77,431	96,357	95,696
Interest income	265	29	568	68	1,055	93
Interest expense	10,217	10,952	31,219	30,600	42,139	40,217
Income from AFUDC (debt and equity)	535	159	805	310	1,007	374
Net Income	25,542	25,184	46,054	46,281	54,745	54,740
Capital Expenditures	41,292	24,093	107,966	76,460	132,684	106,354
Total assets	2,066,555		2,066,555		2,066,555		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	152,730	157,619	396,546	406,308	514,515	523,177
Depreciation and amortization	13,757	13,418	41,086	45,027	54,295	60,983
Federal and state income taxes	15,564	15,703	27,497	26,676	32,464	29,959
Operating income	34,517	35,359	72,594	71,430	89,754	87,258
Interest income	261	30	549	68	1,035	97
Interest expense	9,328	10,047	28,530	27,865	38,525	36,564
Income from AFUDC (debt and equity)	532	158	800	308	1,000	366
Net Income	25,705	25,276	44,569	43,369	51,870	50,340
Capital Expenditures	39,794	20,800	103,361	70,685	126,176	96,532
Total assets	1,980,356		1,980,356		1,980,356		1,931,320
Gas
Statement of Income Information, balance sheet information and capital expenditures of business segments
Revenues	4,999	5,052	26,486	33,344	39,571	47,750
Depreciation and amortization	895	873	2,675	2,617	3,552	3,465
Federal and state income taxes	(232)	(257)	226	1,111	791	1,751
Operating income	532	517	3,120	4,639	4,995	6,631
Interest income	88	75	255	204	310	269
Interest expense	975	979	2,928	2,933	3,906	3,915
Income from AFUDC (debt and equity)	3	1	5	2	7	8
Net Income	(399)	(427)	326	1,792	1,244	2,840
Capital Expenditures	783	1,841	2,352	2,875	3,599	6,480
Total assets	145,650		145,650		145,650		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	1,621	1,761	5,389	4,897	7,249	6,584
Depreciation and amortization	456	466	1,350	1,334	1,822	1,792
Federal and state income taxes	145	206	713	689	1,003	960
Operating income	233	574	1,140	1,362	1,608	1,807
Interest income	2		3		4
Interest expense		2		6	2	11
Net Income	236	335	1,159	1,120	1,631	1,560
Capital Expenditures	715	1,452	2,253	2,900	2,909	3,342
Total assets	27,837		27,837		27,837		26,038
Eliminations
Statement of Income Information, balance sheet information and capital expenditures of business segments
Revenues	(148)	(148)	(444)	(444)	(592)	(592)
Interest income	(86)	(76)	(239)	(204)	(294)	(273)
Interest expense	(86)	(76)	(239)	(204)	(294)	(273)
Total assets	$ (87,288)		$ (87,288)		$ (87,288)		$ (81,420)