Electric utility segment Liquefied Natural Gas (Details)	May. 31, 2015 t
Electric utility subsidiary [Abstract]
Liquefaction capacity annual purchases, first tranche	700,000
Purchase agreement, period of first tranche	5 years
Liquefaction capacity annual purchases, second tranche	600,000
Purchase agreement, period of second tranche	5 years
Liquefaction capacity annual purchases, third tranche	500,000
Purchase agreement, period of third tranche	10 years
Annual capacity purchases, year two	700,000
Annual capacity purchases, year three	700,000
Annual capacity purchases, year four	700,000
Annual capacity purchases, year five	700,000
Annual capacity purchases, year seven	600,000
Annual capacity purchases, year eight	600,000
Annual capacity purchases, year nine	600,000
Annual capacity purchases, year ten	600,000
Annual capacity purchases, year twelve	500,000
Annual capacity purchases, year thirteen	500,000
Annual capacity purchases, year fourteen	500,000
Annual capacity purchases, year fifteen	500,000
Annual capacity purchases, year sixteen	500,000
Annual capacity purchases, year seventeen	500,000
Annual capacity purchases, year eighteen	500,000
Annual capacity purchases, year nineteen	500,000
Annual capacity purchases, year twenty	500,000