Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2012 town segment	Mar. 31, 2011	Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2011
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	$ 137,144	$ 150,728	$ 563,286	$ 552,110
Depreciation and amortization	14,935	17,333	61,138	62,804
Federal and state income taxes	6,199	7,245	33,252	27,844
Operating income	20,810	21,848	95,897	86,265
Interest income	179	23	712	129
Interest expense	10,941	9,711	42,751	40,633
Income from AFUDC (debt and equity)	98	23	586	6,042
Net Income	9,804	11,922	52,854	50,732
Capital Expenditures	35,748	17,557	119,368	94,737
Total assets	2,026,459		2,026,459		2,021,835
Goodwill	39,492		39,492		39,492
Electric
Statement of Operations Information, balance sheet information and capital expenditures of business segments
Revenues	119,726	128,360	515,641	499,042
Depreciation and amortization	13,569	16,028	55,778	57,710
Federal and state income taxes	5,187	5,633	31,197	25,278
Operating income	18,243	18,276	88,557	78,319
Interest income	170	22	702	148
Interest expense	10,027	8,801	39,086	37,040
Income from AFUDC (debt and equity)	98	23	583	6,023
Net Income	8,173	9,301	49,542	46,564
Capital Expenditures	34,079	16,850	110,728	87,659
Total assets	1,940,782		1,940,782		1,931,320
Gas
Statement of Operations Information, balance sheet information and capital expenditures of business segments
Revenues	15,683	20,989	41,124	47,314
Depreciation and amortization	919	873	3,539	3,397
Federal and state income taxes	697	1,381	991	1,561
Operating income	2,054	3,195	5,374	6,287
Interest income	72	71	260	348
Interest expense	977	977	3,910	3,934
Income from AFUDC (debt and equity)			3	19
Net Income	1,119	2,246	1,582	2,534
Capital Expenditures	725	340	4,507	5,293
Total assets	146,331		146,331		145,897
Goodwill	39,492		39,492		39,492
Other.
Statement of Operations Information, balance sheet information and capital expenditures of business segments
Revenues	1,883	1,527	7,113	6,346
Depreciation and amortization	447	432	1,821	1,697
Federal and state income taxes	315	231	1,064	1,005
Operating income	513	377	1,966	1,659
Interest expense		3	5	26
Net Income	512	375	1,730	1,634
Capital Expenditures	944	367	4,133	1,785
Total assets	27,047		27,047		26,038
Eliminations
Statement of Operations Information, balance sheet information and capital expenditures of business segments
Revenues	(148)	(148)	(592)	(592)
Interest income	(63)	(70)	(250)	(367)
Interest expense	(63)	(70)	(250)	(367)
Total assets	$ (87,701)		$ (87,701)		$ (81,420)