Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014 segment town	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013
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	$ 171,512	$ 157,486	$ 500,967	$ 445,272	$ 650,025	$ 574,392
Depreciation and amortization	18,550	17,735	54,648	51,471	72,483	66,807
Federal and state income taxes	13,714	14,150	32,484	28,681	41,296	34,402
Operating income	31,709	32,896	80,698	75,864	104,497	95,231
Interest income	4	5	48	522	92	926
Interest expense	10,384	10,353	31,115	31,107	41,487	41,356
Income from AFUDC (debt and equity)	2,771	1,734	7,115	3,904	9,151	5,027
Net income	23,892	23,996	55,991	48,283	71,154	57,910
Capital Expenditures	62,209	46,126	159,117	118,834	201,750	157,155
Total assets	2,255,326		2,255,326		2,255,326		2,145,045
Goodwill	39,492		39,492		39,492		39,492
Operating segments | Electric
Statement of Income Information, balance sheet information and capital expenditures of business segments
Revenues	164,500	150,370	458,355	406,158	588,611	520,265
Depreciation and amortization	17,127	16,328	50,493	47,216	66,937	61,441
Federal and state income taxes	13,560	13,939	30,138	26,882	37,735	31,651
Operating income	30,380	31,589	74,091	70,097	94,978	86,948
Interest income	2	1	34	498	72	895
Interest expense	9,424	9,380	28,245	28,273	37,655	37,610
Income from AFUDC (debt and equity)	2,769	1,722	7,036	3,883	9,062	5,002
Net income	23,561	23,652	52,271	45,373	65,501	53,435
Capital Expenditures	60,151	45,164	151,764	114,734	191,690	151,490
Total assets	2,143,602		2,143,602		2,143,602		2,034,234
Operating segments | Gas
Statement of Income Information, balance sheet information and capital expenditures of business segments
Revenues	4,989	4,952	36,587	33,222	53,406	46,585
Depreciation and amortization	966	928	2,790	2,778	3,720	3,702
Federal and state income taxes	(293)	(369)	1,065	707	1,842	1,269
Operating income	647	364	4,565	4,004	6,735	5,889
Interest income	4	4	23	109	29	177
Interest expense	968	973	2,893	2,926	3,857	3,902
Income from AFUDC (debt and equity)	2	12	79	21	89	25
Net income	(322)	(599)	1,712	1,136	2,932	2,066
Capital Expenditures	1,628	628	6,084	2,824	7,679	4,043
Total assets	122,153		122,153		122,153		123,736
Goodwill	39,492		39,492		39,492		39,492
Operating segments | Other
Statement of Income Information, balance sheet information and capital expenditures of business segments
Revenues	2,343	2,819	6,985	6,844	9,288	8,641
Depreciation and amortization	457	479	1,365	1,477	1,826	1,664
Federal and state income taxes	447	580	1,281	1,092	1,719	1,482
Operating income	682	943	2,062	1,763	2,784	2,394
Interest income	6		14	7	16	10
Net income	653	943	2,008	1,774	2,721	2,409
Capital Expenditures	430	334	1,269	1,276	2,381	1,622
Total assets	33,721		33,721		33,721		31,306
Eliminations
Statement of Income Information, balance sheet information and capital expenditures of business segments
Revenues	(320)	(655)	(960)	(952)	(1,280)	(1,099)
Interest income	(8)	0	(23)	(92)	(25)	(156)
Interest expense	(8)	0	(23)	(92)	(25)	(156)
Total assets	$ (44,150)		$ (44,150)		$ (44,150)		$ (44,231)