Regulatory matters and revenues subject to refund (Details) (USD $)	6 Months Ended	9 Months Ended	6 Months Ended	12 Months Ended	6 Months Ended
	Jun. 30, 2013 MTPSC [Member]	Sep. 30, 2012 MTPSC [Member]	Jun. 30, 2013 SDPUC [Member]	Dec. 31, 2012 SDPUC [Member]	Jun. 30, 2013 NDPSC [Member]
Regulatory matters and revenues subject to refund [Line Items]
Total gas rate increase requested		$ 3,500,000		$ 1,500,000
Percent above current rates requested		5.90%		3.30%
Amount of requested interim increase in annual gas rates		1,700,000	1,500,000
Requested percentage of interim gas rate increase		2.90%
Interim rate increase (decrease), amount	850,000
Estimated project cost					$ 26,100,000