Commitments (Details) (Long-term transportation contracts [Member], ING Assets acquisition [Member], USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2013 Dekatherm
Commitments (Textual)
Liability related to firm transportation contracts assumed	$ 2.8
For 2013 [Member]
Commitments (Textual)
Capacity levels (Dekatherms per day)	8,000
2014 through May 2015 [Member]
Commitments (Textual)
Capacity levels (Dekatherms per day)	4,590
June 2015 through 2017 [Member]
Commitments (Textual)
Capacity levels (Dekatherms per day)	3,300
Demand charges (in dollars per dekatherm)	0.65
2018 through April 2036 [Member]
Commitments (Textual)
Capacity levels (Dekatherms per day)	1,000
Demand charges (in dollars per dekatherm)	0.22
Maximum [Member] | For 2013 [Member]
Commitments (Textual)
Demand charges (in dollars per dekatherm)	1.40
Maximum [Member] | 2014 through May 2015 [Member]
Commitments (Textual)
Demand charges (in dollars per dekatherm)	0.65
Minimum [Member] | For 2013 [Member]
Commitments (Textual)
Demand charges (in dollars per dekatherm)	0.22
Minimum [Member] | 2014 through May 2015 [Member]
Commitments (Textual)
Demand charges (in dollars per dekatherm)	0.22