Segment Information (details) (USD $) In Thousands, unless otherwise specified	3 Months Ended									6 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 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, 2013	Sep. 30, 2012	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Segment Reporting [Abstract]
Revenues	$ 88,195		$ 87,501								$ 497,770	$ 501,131
Depreciation and Amortization	18,737		18,281								56,474	54,330
Operating Income (Loss)	(3,787)		(4,796)								83,502	85,709
Net Income (Loss) Attributable to Parent	(8,233)	28,147	(10,879)	1,227	40,284	28,766	(8,527)	1,985	40,820	41,511	31,532	30,632	58,779	63,044	72,013
Payments to Acquire Property, Plant, and Equipment	31,232		39,328								86,287	100,880
Assets	2,805,913	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	2,805,913	2,685,148	2,813,120	2,742,718
Utility Margin [Abstract]
Revenues	88,195		87,501								497,770	501,131
Cost of gas	33,655		37,570								235,156	241,823
Utility margin	47,050		42,331								239,151	236,921
Utility Segment [Member]
Segment Reporting [Abstract]
Revenues	80,705		79,901								474,307	478,744
Depreciation and Amortization	17,118		16,661								51,617	49,477
Operating Income (Loss)	(7,293)		(8,439)								72,372	76,072
Net Income (Loss) Attributable to Parent	(9,605)		(12,174)								27,083	27,424
Payments to Acquire Property, Plant, and Equipment	30,805		38,577								85,327	99,019
Assets	2,502,688	2,505,655	2,381,659								2,502,688	2,381,659	2,505,655
Utility Margin [Abstract]
Revenues	80,705		79,901								474,307	478,744
Gas Storage Segment [Member]
Segment Reporting [Abstract]
Revenues	7,434		7,544								23,295	22,219
Depreciation and Amortization	1,619		1,620								4,857	4,853
Operating Income (Loss)	3,556		3,624								11,138	9,567
Net Income (Loss) Attributable to Parent	1,407		1,255								4,495	3,185
Payments to Acquire Property, Plant, and Equipment	427		751								960	1,861
Assets	287,317	291,568	287,687								287,317	287,687	291,568
Utility Margin [Abstract]
Revenues	7,434		7,544								23,295	22,219
Other Segment [Member]
Segment Reporting [Abstract]
Revenues	56		56								168	168
Depreciation and Amortization	0		0								0	0
Operating Income (Loss)	(50)		19								(8)	70
Net Income (Loss) Attributable to Parent	(35)		40								(46)	23
Payments to Acquire Property, Plant, and Equipment	0		0								0	0
Assets	15,908	15,897	15,802								15,908	15,802	15,897
Utility Margin [Abstract]
Revenues	$ 56		$ 56								$ 168	$ 168