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Regulatory matters and revenues subject to refund (Details) (USD $)
 0 Months Ended
Apr. 12, 2013
MTPSC [Member]
Sep. 26, 2012
MTPSC [Member]
Oct. 23, 2013
SDPUC [Member]
Jun. 19, 2013
SDPUC [Member]
Dec. 12, 2012
SDPUC [Member]
Oct. 23, 2013
NDPSC [Member]
Sep. 30, 2013
NDPSC [Member]
Sep. 18, 2013
NDPSC [Member]
Oct. 31, 2013
FERC [Member]
Regulatory matters and revenues subject to refund [Line Items]                  
Total gas rate increase requested   $ 3,500,000     $ 1,500,000     $ 6,800,000  
Percent above current rates requested   5.90%     3.30%     6.40%  
Amount of requested interim increase in annual gas rates   1,700,000   1,500,000       4,500,000  
Requested percentage of interim gas rate increase   2.90%           4.20%  
Interim rate increase (decrease), amount 850,000         4,300,000      
Settlement stipulation     900,000            
Settlement stipulation percent     2.00%            
Estimated project cost             27,700,000    
Settlement agreement, amount           4,300,000      
Settlement agreement, percent           4.00%      
Rate change application annual increase, amount                 28,900,000
Increased investment                 $ 312,000,000