General Information (Details)	Sep. 30, 2015	Jun. 29, 2015
General information
Equity method investment, ownership interest (as a percent)		60.00%
ATC
General information
Equity method investment, ownership interest (as a percent)	60.00%	26.20%
Electric
General information
Number of customers	1,600,000
Natural gas
General information
Number of customers	2,800,000