Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Segment Reporting [Abstract]
Revenues	$ 138,280	$ 133,169	$ 399,945	$ 426,555
Depreciation and Amortization	20,230	19,709	40,341	39,298
Operating Income (Loss)	12,914	13,266	65,915	88,294
Net Income (Loss) Attributable to Parent	2,197	1,071	30,683	38,955
Payments to Acquire Property, Plant, and Equipment	30,937	26,901	58,072	52,489
Assets	2,931,879	2,787,114	2,931,879	2,787,114	$ 3,064,945
Utility Margin [Abstract]
Utility operating revenues	138,280	133,169	399,945	426,555
Less: Utility cost of gas	62,176	58,280	187,881	213,481
Operating Segments [Member] | Utility Segment [Member]
Segment Reporting [Abstract]
Revenues	132,891	128,075	389,197	413,570
Depreciation and Amortization	18,602	18,087	37,077	36,054
Operating Income (Loss)	12,163	13,735	64,043	85,192
Net Income (Loss) Attributable to Parent	2,245	2,205	30,580	38,224
Payments to Acquire Property, Plant, and Equipment	30,464	26,726	56,273	52,076
Assets	2,646,457	2,487,771	2,646,457	2,487,771	2,775,011
Utility Margin [Abstract]
Utility operating revenues	132,891	128,075	389,197	413,570
Less: Utility cost of gas	62,176	58,280	187,881	213,481
Utility margin	70,715	69,795	201,316	200,089
Operating Segments [Member] | Gas Storage Segment [Member]
Segment Reporting [Abstract]
Revenues	5,333	5,038	10,636	12,873
Depreciation and Amortization	1,628	1,622	3,264	3,244
Operating Income (Loss)	739	(485)	1,794	3,068
Net Income (Loss) Attributable to Parent	(86)	(1,157)	28	470
Payments to Acquire Property, Plant, and Equipment	473	175	1,799	413
Assets	270,509	282,939	270,509	282,939	273,813
Utility Margin [Abstract]
Utility operating revenues	5,333	5,038	10,636	12,873
Operating Segments [Member] | Other Segment [Member]
Segment Reporting [Abstract]
Revenues	56	56	112	112
Depreciation and Amortization	0	0	0	0
Operating Income (Loss)	12	16	78	34
Net Income (Loss) Attributable to Parent	38	23	75	261
Payments to Acquire Property, Plant, and Equipment	0	0	0	0
Assets	14,913	16,404	14,913	16,404	$ 16,121
Utility Margin [Abstract]
Utility operating revenues	$ 56	$ 56	$ 112	$ 112