Segment Information (details) (USD $) In Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	Dec. 31, 2010
Segment Reporting Abstract
Net operating revenues	$ 47,783	$ 46,211	$ 249,523	$ 249,330
Depreciation and amortization	17,449	16,003	52,304	47,930
Income from operations	(5,552)	(3,364)	84,963	97,964
Net income	(8,312)	(7,420)	34,654	43,076
Total assets	2,567,840	2,481,377	2,567,840	2,481,377	2,616,616
Utility Segment [Member]
Segment Reporting Abstract
Net operating revenues	41,034	41,258	230,244	233,670
Depreciation and amortization	15,875	15,668	47,735	46,925
Income from operations	(8,029)	(6,858)	77,762	85,995
Net income	(9,518)	(9,123)	31,702	36,410
Total assets	2,291,531	2,192,557	2,291,531	2,192,557	2,310,388
Gas Storage Segment [Member]
Segment Reporting Abstract
Net operating revenues	6,710	4,906	19,211	15,523
Depreciation and amortization	1,574	335	4,569	1,005
Income from operations	2,458	3,474	7,191	11,910
Net income	1,160	1,782	3,163	6,405
Total assets	253,478	266,022	253,478	266,022	282,945
Other Segment [Member]
Segment Reporting Abstract
Net operating revenues	39	47	68	137
Depreciation and amortization	0	0	0	0
Income from operations	19	20	10	59
Net income	46	(79)	(211)	261
Total assets	$ 22,831	$ 22,798	$ 22,831	$ 22,798	$ 23,283