Regulatory matters and revenues subject to refund (Details) (USD $)	Apr. 12, 2013 MTPSC [Member]	Sep. 26, 2012 MTPSC [Member]	Dec. 21, 2012 SDPUC [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
Requested percentage of interim gas rate increase		2.90%
Authorized interim increase	$ 850,000