Segment Information (details) (USD $) In Thousands, unless otherwise specified	3 Months Ended									6 Months Ended	9 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Jun. 30, 2012	Sep. 30, 2012	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Segment Reporting [Abstract]
Revenues	$ 277,861				$ 309,639
Depreciation and Amortization	18,807				17,950
Operating Income (Loss)	74,206				78,666
Net Income (Loss) Attributable to Parent	37,639	28,147	(10,879)	1,227	40,284	28,766	(8,527)	1,985	40,820	41,511	30,632	58,779	63,044	72,013
Payments to Acquire Property, Plant, and Equipment	22,674				20,447
Assets	2,806,499	2,813,120	2,685,148	2,630,450	2,722,873	2,742,718	2,564,775	2,519,284	2,569,186	2,630,450	2,685,148	2,813,120	2,742,718
Utility Margin [Abstract]
Revenues	277,861				309,639
Cost of Revenue	142,359				169,755
Utility Margin	127,300				133,150
Utility Segment [Member]
Segment Reporting [Abstract]
Revenues	269,659				302,905
Depreciation and Amortization	17,188				16,338
Operating Income (Loss)	70,228				75,964
Net Income (Loss) Attributable to Parent	36,031				39,468
Payments to Acquire Property, Plant, and Equipment	22,388				19,656
Assets	2,501,724	2,505,655			2,420,194							2,505,655
Utility Margin [Abstract]
Revenues	269,659				302,905
Gas Storage Segment [Member]
Segment Reporting [Abstract]
Revenues	8,146				6,679
Depreciation and Amortization	1,619				1,612
Operating Income (Loss)	3,957				2,679
Net Income (Loss) Attributable to Parent	1,636				806
Payments to Acquire Property, Plant, and Equipment	286				791
Assets	288,795	291,568			286,756							291,568
Utility Margin [Abstract]
Revenues	8,146				6,679
Other Segment [Member]
Segment Reporting [Abstract]
Revenues	56				55
Depreciation and Amortization	0				0
Operating Income (Loss)	21				23
Net Income (Loss) Attributable to Parent	(28)				10
Payments to Acquire Property, Plant, and Equipment	0				0
Assets	15,980	15,897			15,923							15,897
Utility Margin [Abstract]
Revenues	$ 56				$ 55