Derivative Instruments and Hedging Activities (Commodity Volumes) (Details)	Mar. 31, 2015 MWh
Sales Contracts [Member] | Power [Member] | 2015 [Member]
Commodity Volumetric Activity [Line Items]
Notional amount (in units)	30,874,062	[1],[2]
Sales Contracts [Member] | Power [Member] | 2016 [Member]
Commodity Volumetric Activity [Line Items]
Notional amount (in units)	8,521,382	[1]
Sales Contracts [Member] | Power [Member] | 2017 [Member]
Commodity Volumetric Activity [Line Items]
Notional amount (in units)	248,329	[1]
Sales Contracts [Member] | Capacity [Member] | 2015 [Member]
Commodity Volumetric Activity [Line Items]
Notional amount (in units)	3,998	[1],[2]
Sales Contracts [Member] | Capacity [Member] | 2016 [Member]
Commodity Volumetric Activity [Line Items]
Notional amount (in units)	878	[1]
Purchase Contracts [Member] | Power [Member] | Thereafter [Member]
Commodity Volumetric Activity [Line Items]
Notional amount (in units)	2,236,333	[1]
Purchase Contracts [Member] | Capacity [Member] | 2017 [Member]
Commodity Volumetric Activity [Line Items]
Notional amount (in units)	6	[1]
Purchase Contracts [Member] | Capacity [Member] | Thereafter [Member]
Commodity Volumetric Activity [Line Items]
Notional amount (in units)	3	[1]
Purchase Contracts [Member] | Gas [Member] | 2015 [Member]
Commodity Volumetric Activity [Line Items]
Notional amount (in units)	157,995,389	[1],[2]
Purchase Contracts [Member] | Gas [Member] | 2016 [Member]
Commodity Volumetric Activity [Line Items]
Notional amount (in units)	87,545,701	[1]
Purchase Contracts [Member] | Gas [Member] | 2017 [Member]
Commodity Volumetric Activity [Line Items]
Notional amount (in units)	13,742,416	[1]
Purchase Contracts [Member] | Gas [Member] | Thereafter [Member]
Commodity Volumetric Activity [Line Items]
Notional amount (in units)	20,314,625	[1]
Purchase Contracts [Member] | Financial Transmission Rights [Member] | 2015 [Member]
Commodity Volumetric Activity [Line Items]
Notional amount (in units)	532	[1],[2]
Purchase Contracts [Member] | Oil [Member] | 2015 [Member]
Commodity Volumetric Activity [Line Items]
Notional amount (in units)	300,328	[1],[2]
Purchase Contracts [Member] | Oil [Member] | 2016 [Member]
Commodity Volumetric Activity [Line Items]
Notional amount (in units)	387,429	[1]
Purchase Contracts [Member] | Oil [Member] | 2017 [Member]
Commodity Volumetric Activity [Line Items]
Notional amount (in units)	257,483	[1]
Purchase Contracts [Member] | Oil [Member] | Thereafter [Member]
Commodity Volumetric Activity [Line Items]
Notional amount (in units)	60,000	[1]

[1] Volumes for option contracts factor in the probability of an option being exercised and may be less than the notional amount of the option.

[2] Represents balance of the current year.