Electric utility subsidiary - Liquefied natural gas (Details) - t		3 Months Ended
	Aug. 08, 2014	Mar. 31, 2015	Aug. 31, 2014
Electric utility subsidiary [Abstract]
Term of agreement	15 years
Liquefaction capacity purchases, year one			800,000
Purchase agreement, period of first tranche		5 years
Purchase agreement, period of second tranche		5 years
Purchase agreement, period of third tranche		5 years
Liquefaction capacity purchases, year two			800,000
Liquefaction capacity purchases, year three			800,000
Liquefaction capacity purchases, year four			800,000
Liquefaction capacity purchases, year five			800,000
Liquefaction capacity purchases, year six			700,000
Liquefaction capacity purchases, year seven			700,000
Liquefaction capacity purchases, year eight			700,000
Liquefaction capacity purchases, year nine			700,000
Liquefaction capacity purchases, year ten			700,000
Liquefaction capacity purchases, year eleven			600,000
Liquefaction capacity purchases, year twelve			600,000
Liquefaction capacity purchases, year thirteen			600,000
Liquefaction capacity purchases, year fourteen			600,000
Liquefaction capacity purchases, year fifteen			600,000