Electric utility subsidiary - Liquefied natural gas (Details)	May. 31, 2015 t
Electric utility subsidiary [Abstract]
Liquefaction capacity purchases, year one	700,000
Purchase agreement, period of first tranche	5 years
Purchase agreement, period of second tranche	5 years
Purchase agreement, period of third tranche	10 years
Liquefaction capacity purchases, year two	700,000
Liquefaction capacity purchases, year three	700,000
Liquefaction capacity purchases, year four	700,000
Liquefaction capacity purchases, year five	700,000
Liquefaction capacity purchases, year six	600,000
Liquefaction capacity purchases, year seven	600,000
Liquefaction capacity purchases, year eight	600,000
Liquefaction capacity purchases, year nine	600,000
Liquefaction capacity purchases, year ten	600,000
Liquefaction capacity purchases, year eleven	500,000
Liquefaction capacity purchases, year twelve	500,000
Liquefaction capacity purchases, year thirteen	500,000
Liquefaction capacity purchases, year fourteen	500,000
Liquefaction capacity purchases, year fifteen	500,000
Purchase Agreement, Annual Capacity Purchases, Year Sixteen	500,000
Purchase Agreement, Annual Capacity Purchases, Year Seventeen	500,000
Purchase Agreement, Annual Capacity Purchases, Year Eighteen	500,000
Purchase Agreement, Annual Capacity Purchases, Year Nineteen	500,000
Purchase Agreement, Annual Capacity Purchases, Year Twenty	500,000