Regulatory Matters - Additional Information (Detail) (USD $)	1 Months Ended	9 Months Ended
	Feb. 28, 2009	Sep. 30, 2012	Mar. 31, 2012	Oct. 01, 2010
Schedule Of Regulatory Assets And Liabilities [Line Items]
Surcharge collected from customers			$ 134,000,000
Potential operational penalties		3,000,000
Revenue from electric	249,000,000
Revenue from gas	32,000,000
Revenue from steam service	6,000,000
Potential refund from customers		1,031,000,000
Provisional assessment				21,000,000
Regulatory liability		15,000,000
July 2012 [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Electric base rate increase		19.4
Surcharge payment		15,200,000
July 2013 [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Electric base rate increase		8.8
Surcharge payment		15,200,000
July 2014 [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Electric base rate increase		15.2
Surcharge payment		15,200,000
June 30, 2013 [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Weighted average cost of capital		7.61%
Return on common equity		9.40%
Cost of long term debt		6.07%
Common equity ratio		48.00%
Average base rate		671,000,000
Percentage of revenue allocated to customers		50.00%
Common equity basis points		80
Average net utility plant rates		678,000,000
June 30, 2014 [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Weighted average cost of capital		7.65%
Return on common equity		9.50%
Cost of long term debt		6.07%
Common equity ratio		48.00%
Average base rate		708,000,000
Percentage of revenue allocated to customers		75.00%
Common equity basis points		180
Average net utility plant rates		704,000,000
June 30, 2015 [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Weighted average cost of capital		7.48%
Return on common equity		9.60%
Cost of long term debt		5.64%
Common equity ratio		48.00%
Average base rate		759,000,000
Percentage of revenue allocated to customers		90.00%
Common equity basis points		280
Average net utility plant rates		$ 753,000,000
Maximum [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Common equity ratio		50.00%