Segment Information (details) (USD $) In Thousands, unless otherwise specified	3 Months Ended									6 Months Ended		9 Months Ended	12 Months Ended
	Jun. 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, 2013	Jun. 30, 2012	Sep. 30, 2012	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Segment Reporting [Abstract]
Revenues	$ 131,714			$ 103,991						$ 409,575	$ 413,630
Depreciation and Amortization	18,930			18,099						37,737	36,049
Operating Income (Loss)	13,083			11,839						87,289	90,505
Net Income (Loss) Attributable to Parent	2,126	28,147	(10,879)	1,227	40,284	28,766	(8,527)	1,985	40,820	39,765	41,511	30,632	58,779	63,044	72,013
Payments to Acquire Property, Plant, and Equipment	32,381			41,105						55,055	61,552
Assets	2,772,589	2,813,120	2,685,148	2,630,450	2,722,873	2,742,718	2,564,775	2,519,284	2,569,186	2,772,589	2,630,450	2,685,148	2,813,120	2,742,718
Utility Margin [Abstract]
Revenues	131,714			103,991						409,575	413,630
Cost of gas	59,142			34,498						201,501	204,253
Utility margin	64,801			61,440						192,101	194,590
Utility Segment [Member]
Segment Reporting [Abstract]
Revenues	123,943			95,938						393,602	398,843
Depreciation and Amortization	17,311			16,478						34,499	32,816
Operating Income (Loss)	9,437			8,547						79,665	84,511
Net Income (Loss) Attributable to Parent	657			130						36,688	39,598
Payments to Acquire Property, Plant, and Equipment	32,134			40,786						54,522	60,442
Assets	2,469,320	2,505,655		2,326,919						2,469,320	2,326,919		2,505,655
Utility Margin [Abstract]
Revenues	123,943			95,938						393,602	398,843
Gas Storage Segment [Member]
Segment Reporting [Abstract]
Revenues	7,715			7,996						15,861	14,675
Depreciation and Amortization	1,619			1,621						3,238	3,233
Operating Income (Loss)	3,625			3,264						7,582	5,943
Net Income (Loss) Attributable to Parent	1,452			1,124						3,088	1,930
Payments to Acquire Property, Plant, and Equipment	247			319						533	1,110
Assets	287,341	291,568		287,622						287,341	287,622		291,568
Utility Margin [Abstract]
Revenues	7,715			7,996						15,861	14,675
Other Segment [Member]
Segment Reporting [Abstract]
Revenues	56			57						112	112
Depreciation and Amortization	0			0						0	0
Operating Income (Loss)	21			28						42	51
Net Income (Loss) Attributable to Parent	17			(27)						(11)	(17)
Payments to Acquire Property, Plant, and Equipment	0			0						0	0
Assets	15,928	15,897		15,909						15,928	15,909		15,897
Utility Margin [Abstract]
Revenues	$ 56			$ 57						$ 112	$ 112