Segment Information (details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013
Segment Reporting [Abstract]
Revenues	$ 133,169	$ 131,714	$ 426,555	$ 409,575
Depreciation and Amortization	19,709	18,930	39,298	37,737
Operating Income (Loss)	13,266	13,083	88,294	87,289
Net Income (Loss) Attributable to Parent	1,071	2,126	38,955	39,765
Payments to Acquire Property, Plant, and Equipment	26,901	32,381	52,489	55,055
Assets	2,787,114	2,772,589	2,787,114	2,772,589	2,970,911
Utility Margin [Abstract]
Cost of gas	58,280	59,142	213,481	201,501
Utility margin	69,795	64,801	200,089	192,101
Utility Segment [Member]
Segment Reporting [Abstract]
Revenues	128,075	123,943	413,570	393,602
Depreciation and Amortization	18,087	17,311	36,054	34,499
Operating Income (Loss)	13,735	9,437	85,192	79,665
Net Income (Loss) Attributable to Parent	2,205	657	38,224	36,688
Payments to Acquire Property, Plant, and Equipment	26,726	32,134	52,076	54,522
Assets	2,487,771	2,469,320	2,487,771	2,469,320	2,644,367
Gas Storage Segment [Member]
Segment Reporting [Abstract]
Revenues	5,038	7,715	12,873	15,861
Depreciation and Amortization	1,622	1,619	3,244	3,238
Operating Income (Loss)	(485)	3,625	3,068	7,582
Net Income (Loss) Attributable to Parent	(1,157)	1,452	470	3,088
Payments to Acquire Property, Plant, and Equipment	175	247	413	533
Assets	282,939	287,341	282,939	287,341	310,097
Other Segment [Member]
Segment Reporting [Abstract]
Revenues	56	56	112	112
Depreciation and Amortization	0	0	0	0
Operating Income (Loss)	16	21	34	42
Net Income (Loss) Attributable to Parent	23	17	261	(11)
Payments to Acquire Property, Plant, and Equipment	0	0	0	0
Assets	$ 16,404	$ 15,928	$ 16,404	$ 15,928	$ 16,447