Segment Information (details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2013
Segment Reporting [Abstract]
Revenues	$ 293,386	$ 277,861
Depreciation and Amortization	19,589	18,807
Operating Income (Loss)	75,028	74,206
Net Income (Loss) Attributable to Parent	37,884	37,639
Payments to Acquire Property, Plant, and Equipment	25,588	22,674
Assets	2,830,384	2,806,499	2,970,911
Utility Margin [Abstract]
Cost of gas	155,201	142,359
Utility margin	130,294	127,300
Utility Segment [Member]
Segment Reporting [Abstract]
Revenues	285,495	269,659
Depreciation and Amortization	17,967	17,188
Operating Income (Loss)	71,457	70,228
Net Income (Loss) Attributable to Parent	36,019	36,031
Payments to Acquire Property, Plant, and Equipment	25,350	22,388
Assets	2,506,930	2,501,724	2,644,367
Gas Storage Segment [Member]
Segment Reporting [Abstract]
Revenues	7,835	8,146
Depreciation and Amortization	1,622	1,619
Operating Income (Loss)	3,553	3,957
Net Income (Loss) Attributable to Parent	1,627	1,636
Payments to Acquire Property, Plant, and Equipment	238	286
Assets	307,055	288,795	310,097
Other Segment [Member]
Segment Reporting [Abstract]
Revenues	56	56
Depreciation and Amortization	0	0
Operating Income (Loss)	18	21
Net Income (Loss) Attributable to Parent	238	(28)
Payments to Acquire Property, Plant, and Equipment	0	0
Assets	$ 16,399	$ 15,980	$ 16,447