Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2013 town segment	Mar. 31, 2012	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
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	$ 151,140	$ 137,144	$ 571,094	$ 563,286
Depreciation and amortization	16,100	14,935	61,612	61,138
Federal and state income taxes	7,482	6,199	35,441	33,252
Operating income	21,858	20,810	97,269	95,897
Interest income	508	179	1,300	712
Interest expense	10,250	10,941	40,776	42,751
Income from AFUDC (debt and equity)	831	98	2,661	586
Net Income	12,630	9,804	58,506	52,854
Capital Expenditures	41,394	35,748	152,895	119,368
Total assets	2,135,773		2,135,773		2,126,369
Goodwill	39,492		39,492		39,492
Electric
Statement of Income Information, balance sheet information and capital expenditures of business segments
Revenues	128,762	119,726	519,690	515,641
Depreciation and amortization	14,682	13,569	56,424	55,778
Federal and state income taxes	5,995	5,187	33,074	31,197
Operating income	18,515	18,243	89,717	88,557
Interest income	495	170	1,270	702
Interest expense	9,337	10,027	37,175	39,086
Income from AFUDC (debt and equity)	830	98	2,651	583
Net Income	10,223	8,173	54,680	49,542
Capital Expenditures	40,221	34,079	147,220	110,728
Total assets	2,046,653		2,046,653		2,034,399
Gas
Statement of Income Information, balance sheet information and capital expenditures of business segments
Revenues	20,493	15,683	44,659	41,124
Depreciation and amortization	924	919	3,603	3,539
Federal and state income taxes	1,206	697	1,298	991
Operating income	2,895	2,054	5,845	5,374
Interest income	72	72	323	260
Interest expense	977	977	3,906	3,910
Income from AFUDC (debt and equity)	1		10	3
Net Income	1,950	1,119	2,087	1,582
Capital Expenditures	733	725	3,579	4,507
Total assets	153,911		153,911		148,814
Other
Statement of Income Information, balance sheet information and capital expenditures of business segments
Revenues	2,033	1,883	7,337	7,113
Depreciation and amortization	494	447	1,585	1,821
Federal and state income taxes	281	315	1,069	1,064
Operating income	448	513	1,707	1,966
Interest income	5		12
Interest expense				5
Net Income	457	512	1,739	1,730
Capital Expenditures	440	944	2,096	4,133
Total assets	29,090		29,090		28,871
Eliminations
Statement of Income Information, balance sheet information and capital expenditures of business segments
Revenues	(148)	(148)	(592)	(592)
Interest income	(64)	(63)	(305)	(250)
Interest expense	(64)	(63)	(305)	(250)
Total assets	$ (93,881)		$ (93,881)		$ (85,715)