Regulatory Matters (Details Textuals) (USD $)	3 Months Ended	12 Months Ended			3 Months Ended		9 Months Ended		12 Months Ended		9 Months Ended	12 Months Ended
	Mar. 31, 2012	Dec. 31, 2012 people	Dec. 31, 2011	Jun. 30, 2010	Mar. 31, 2012 Electric [Member]	Mar. 31, 2011 Electric [Member]	Sep. 30, 2012 Electric [Member] kWh	Sep. 30, 2011 Electric [Member]	Dec. 31, 2012 Electric [Member]	Jun. 30, 2012 Electric [Member]	Sep. 30, 2011 Gas [Member]	Dec. 31, 2012 Gas [Member]	Dec. 31, 2011 Gas [Member]	Sep. 30, 2012 Gas [Member]	Jun. 30, 2012 Gas [Member]
Regulatory Asset and Liability [Line Items]
Probability of Recovery Through Regulatory Rates for Regulatory Assets and Liabilities		$ 0
Time Period Required by Non-Bypassable Securitization Bond Surcharge to Recover Securitized Regulatory Asset		14 years
Amortization Period of Cost to Achieve Deferral		10 years
Deferral of Investigation and Remediation of Costs Associated with Gas Utilities Former MGP Sites		10 years
Amortization Period for Enterprise Business Systems		10 years
Approved Rate Base Relating to the AMI Program										37,000,000
Expenditures Net of Grant Funds for AMI Program										110,000,000
Matching Grant Funds for the AMI Program										60,000,000
Defer the Reversal of RDM Liability							127,000,000
Number of Parties that Appealed the COA Decision Regarding RDM		0
Monthly Amount to Amortize from Liability to Income							10,600,000
EO Plans Performance Incentive Surcharges									8,400,000			3,400,000
Requested Recovery of Enery Optimization Expenses for Future Period Revised								224,000,000			66,000,000
Requested net recovery of RETM and LCT			44,000,000
Requesting Authority to Refund Uncollectible Expense to Customers	9,000,000
Authorized Recovery Amount for CIM									63,000,000
Authorized Recovery Period for CIM									10 months
PSCR Reconciliation Calculating net Under Recovery					148,000,000
Net Under Recovery of Power Suppy Cost Recovery Expense						52,600,000
PSCR Reconciliation Includes an Over-Recovery					3,800,000
Net Over Recovery of Power Supply Cost Recovery Expense								10,500,000
Power Supply Cost Recovery Plan Case Factor Next Year							4.74
Total PSCR Forecasted Expense							1,500,000,000
Recovery of Current Year PSCR							81,000,000
Projected Test Year Period for Gas Rate Case												12 months
Gas Rate Increase to Recover Higher Costs														77,000,000
Gas Self Implementation of Rates												27,000,000
MPSC Approved Increase in Annual Gas Rates												19,900,000
RDM Refund Liability												1,000,000
UETM Net Refund														7,000,000
UTEM Sufficiency Gross Amount Related to Uncollectibles														19,000,000
Under Recovery Relating to Under-Collection of UETM Gross														12,000,000
Time Period to Implementation Credits and Surcharges Related to the UETM												12 months
UETM Accrual for Over-Recovery		20,000,000
RDM Application Proposed Recovery														8,600,000	20,000,000
Time Period to Implement RDM Surcharge												12 months
RDM Revenue Recognized For 2010/2011 Period															5,000,000
RDM Revenue Recognized for 2011/2012 Period															3,000,000
RDM Amount Recognized for the Under-Recovery												5,000,000
RDM Caps for First Reconciliation Period		1.125%
RDM Cap for Second Period Reconciliation		2.25%
Annual Depreciation Expense Increase															12,400,000
GCR Time Period of Reconciliation												12 months	12 months
GCR Reconciliation Over-Recovery														6,000,000
GCR Reconciliation Under-Recovery															6,400,000
Amortization Period of CTA		10 years
Pre Tax Amount of Cost To Achieve Previously Expensed Deferred and Recognized in Income				32,000,000
After Tax Amount of Cost To Achieve Previously Expensed deferred and Recognized in Income				$ 20,000,000