Derivative Instruments and Hedging Activities (Commodity Volumes) (Details)	Sep. 30, 2015 MMBTU MWh MW-M T bbl	[2]
Power | Sales | 2015
Derivative [Line Items]
Volumes (in MWh, MW-Month, MMBtu) | MWh	13,407,010	[1]
Power | Sales | 2016
Derivative [Line Items]
Volumes (in MWh, MW-Month, MMBtu) | MWh	29,872,806
Power | Sales | 2017
Derivative [Line Items]
Volumes (in MWh, MW-Month, MMBtu) | MWh	1,202,348
Power | Sales | Thereafter
Derivative [Line Items]
Volumes (in MWh, MW-Month, MMBtu) | MWh	795,983
Capacity | Sales | 2015
Derivative [Line Items]
Volumes (in MWh, MW-Month, MMBtu)	538	[1]
Capacity | Sales | 2016
Derivative [Line Items]
Volumes (in MWh, MW-Month, MMBtu)	878
Capacity | Purchases | 2017
Derivative [Line Items]
Volumes (in MWh, MW-Month, MMBtu)	6
Capacity | Purchases | Thereafter
Derivative [Line Items]
Volumes (in MWh, MW-Month, MMBtu)	3
Gas | Purchases | 2015
Derivative [Line Items]
Volumes (in MWh, MW-Month, MMBtu) | MMBTU	61,373,321	[1]
Gas | Purchases | 2016
Derivative [Line Items]
Volumes (in MWh, MW-Month, MMBtu) | MMBTU	98,156,219
Gas | Purchases | 2017
Derivative [Line Items]
Volumes (in MWh, MW-Month, MMBtu) | MMBTU	15,586,507
Gas | Purchases | Thereafter
Derivative [Line Items]
Volumes (in MWh, MW-Month, MMBtu) | MMBTU	19,636,898
FTRs | Purchases | 2015
Derivative [Line Items]
Volumes (in MWh, MW-Month, MMBtu)	2,023	[1]
FTRs | Purchases | 2016
Derivative [Line Items]
Volumes (in MWh, MW-Month, MMBtu)	2,889
FTRs | Purchases | 2017
Derivative [Line Items]
Volumes (in MWh, MW-Month, MMBtu)	0
FTRs | Purchases | Thereafter
Derivative [Line Items]
Volumes (in MWh, MW-Month, MMBtu)	0
Oil | Purchases | 2015
Derivative [Line Items]
Volumes (in barrels) | bbl	29,795	[1]
Oil | Purchases | 2016
Derivative [Line Items]
Volumes (in barrels) | bbl	60,000
Oil | Purchases | 2017
Derivative [Line Items]
Volumes (in barrels) | bbl	0
Oil | Purchases | Thereafter
Derivative [Line Items]
Volumes (in barrels) | bbl	0
CRR purchase contracts | Purchases | 2015
Derivative [Line Items]
Volumes (in MWh, MW-Month, MMBtu) | MWh	475,320	[1]
CRR purchase contracts | Purchases | 2016
Derivative [Line Items]
Volumes (in MWh, MW-Month, MMBtu) | MWh	1,393,430
CRR purchase contracts | Purchases | 2017
Derivative [Line Items]
Volumes (in MWh, MW-Month, MMBtu) | MWh	538,584
CRR purchase contracts | Purchases | Thereafter
Derivative [Line Items]
Volumes (in MWh, MW-Month, MMBtu) | MWh	0
Emission Allowances | Purchases | 2015
Derivative [Line Items]
Volumes (tons) | T	2,253,123	[1]
Emission Allowances | Purchases | 2016
Derivative [Line Items]
Volumes (tons) | T	0
Emission Allowances | Purchases | 2017
Derivative [Line Items]
Volumes (tons) | T	0
Emission Allowances | Purchases | Thereafter
Derivative [Line Items]
Volumes (tons) | T	0

[1] Represents balance of the current year.

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