Segment Information (details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2011
Segment Reporting Abstract
Net operating revenues	$ 139,868	$ 134,508
Depreciation and amortization	17,950	17,309
Income from operations	78,666	77,862
Net income	40,607	40,773
Total assets	2,727,262	2,571,553	2,746,574
Utility Segment [Member]
Segment Reporting Abstract
Net operating revenues	133,150	129,162
Depreciation and amortization	16,338	15,914
Income from operations	75,964	76,124
Net income	39,791	40,130
Total assets	2,424,583	2,304,731	2,435,888
Gas Storage Segment [Member]
Segment Reporting Abstract
Net operating revenues	6,679	5,304
Depreciation and amortization	1,612	1,395
Income from operations	2,679	1,716
Net income	806	688
Total assets	286,756	244,403	294,637
Other Segment [Member]
Segment Reporting Abstract
Net operating revenues	39	42
Depreciation and amortization	0	0
Income from operations	23	22
Net income	10	(45)
Total assets	$ 15,923	$ 22,419	$ 16,049