Segment Information (details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011
Segment Reporting Abstract
Net operating revenues	$ 69,479	$ 67,232	$ 209,347	$ 201,740
Depreciation and amortization	18,099	17,546	36,049	34,855
Income from operations	11,839	12,653	90,505	90,515
Net income	1,409	2,193	42,016	42,966
Total assets	2,635,141	2,521,994	2,635,141	2,521,994	2,746,574
Utility Segment [Member]
Segment Reporting Abstract
Net operating revenues	61,440	60,048	194,590	189,210
Depreciation and amortization	16,478	15,946	32,816	31,860
Income from operations	8,547	9,667	84,511	85,791
Net income	312	1,090	40,103	41,220
Total assets	2,331,610	2,247,349	2,331,610	2,247,349	2,435,888
Gas Storage Segment [Member]
Segment Reporting Abstract
Net operating revenues	7,996	7,197	14,675	12,501
Depreciation and amortization	1,621	1,600	3,233	2,995
Income from operations	3,264	3,017	5,943	4,733
Net income	1,124	1,315	1,930	2,003
Total assets	287,622	252,393	287,622	252,393	294,637
Other Segment [Member]
Segment Reporting Abstract
Net operating revenues	43	(13)	82	29
Depreciation and amortization	0	0	0	0
Income from operations	28	(31)	51	(9)
Net income	(27)	(212)	(17)	(257)
Total assets	$ 15,909	$ 22,252	$ 15,909	$ 22,252	$ 16,049