Segment Information (details) (USD $) In Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010
Segment Reporting Abstract
Net operating revenues	$ 67,232	$ 72,193	$ 201,740	$ 203,119
Depreciation and amortization	17,546	16,026	34,855	31,927
Income from operations	12,653	20,218	90,515	101,328
Net income	2,193	6,888	42,966	50,496
Total assets	2,521,994	2,395,561	2,521,994	2,395,561	2,616,616
Utility Segment [Member]
Segment Reporting Abstract
Net operating revenues	60,048	66,939	189,210	192,412
Depreciation and amortization	15,946	15,691	31,860	31,257
Income from operations	9,667	16,271	85,791	92,853
Net income	1,090	4,641	41,220	45,533
Total assets	2,247,349	2,143,138	2,247,349	2,143,138	2,310,388
Gas Storage Segment [Member]
Segment Reporting Abstract
Net operating revenues	7,197	5,206	12,501	10,617
Depreciation and amortization	1,600	335	2,995	670
Income from operations	3,017	3,925	4,733	8,436
Net income	1,315	2,122	2,003	4,623
Total assets	252,393	229,919	252,393	229,919	282,945
Other Segment [Member]
Segment Reporting Abstract
Net operating revenues	(13)	48	29	90
Depreciation and amortization	0	0	0	0
Income from operations	(31)	22	(9)	39
Net income	(212)	125	(257)	340
Total assets	$ 22,252	$ 22,504	$ 22,252	$ 22,504	$ 23,283