Derivatives (Volumes Of Outstanding Derivative Contracts, In Megawatt Hours Unless Otherwise Specified) (Detail)	3 Months Ended		12 Months Ended
	Sep. 30, 2014		Dec. 31, 2013
Derivative [Line Items]
Derivatives expiration, lower (year)	2019 years		2019 years
Derivatives expiration, higher (year)	2023 years		2022 years
Forwards And Swaps [Member] | Natural Gas [Member]
Derivative [Line Items]
Less Than 1 Year	218,107,731		243,213,288
Greater Than 1 Year but Less Than 3 Years	70,621,174		79,735,000
Greater Than 3 Years but Less Than 5 Years	3,530,000	[1]	8,892,500
Greater Than 5 Years	0	[1]	0	[2]
Forwards And Swaps [Member] | Electricity [Member]
Derivative [Line Items]
Less Than 1 Year	1,364,184	[3],[4]	2,537,023
Greater Than 1 Year but Less Than 3 Years	1,956,498	[3],[4]	2,009,505
Greater Than 3 Years but Less Than 5 Years	1,574,588	[3],[4]	2,008,046
Greater Than 5 Years	969,478	[3],[4],[5]	1,534,695	[6]
Options [Member] | Natural Gas [Member]
Derivative [Line Items]
Less Than 1 Year	116,213,741	[7]	169,123,208	[8],[9]
Greater Than 1 Year but Less Than 3 Years	62,844,400	[7]	87,689,708
Greater Than 3 Years but Less Than 5 Years	0	[7]	3,450,000	[8]
Greater Than 5 Years	0	[10],[3],[5]	0	[11]
Congestion Revenue Rights [Member] | Electricity [Member]
Derivative [Line Items]
Less Than 1 Year	55,886,532	[3],[4]	73,510,440
Greater Than 1 Year but Less Than 3 Years	87,533,830	[3],[4]	83,747,782
Greater Than 3 Years but Less Than 5 Years	41,126,312	[3],[4]	63,718,517
Greater Than 5 Years	22,755,431	[4],[7]	29,945,852

[1] Includes the effect of state flow-through ratemaking treatment. Additionally, during the three months ended September 30, 2013, the Utility recorded an adjustment to state income taxes in connection with an IRS settlement.

[2] The proceeds were used for general corporate purposes, including the repayment of a portion of the Utility's outstanding commercial paper.

[3] Amounts shown reflect the total gross derivative volumes by commodity type that are expected to settle in each period.

[4] Million British Thermal Units.

[5] Derivatives in this category expire between 2019 and 2023.

[6] Derivatives in this category expire between 2019 and 2022.

[7] Represents effect of federal flow-through ratemaking treatment including those deductions related to repairs and certain other property-related costs discussed above.

[8] The proceeds were used to repay the 4.80% Senior Notes, in the principal outstanding amount of $539 million, to fund capital expenditures, and for general corporate purposes.

[9] Primarily relates to research and development tax credits and other timing differences.

[10] Amounts shown are for the combined positions of the electric fuels and core gas portfolios.

[11] The proceeds were used for general corporate purposes, including the repayment of a portion of the Utility's outstanding commercial paper, and to fund capital expenditures.