Derivative Instruments and Hedging Activities (Commodity Volumes) (Details)	Jun. 30, 2016 MMBTU MW-M MWh T bbl	[2]
Power | Sales | 2016
Derivative [Line Items]
Volumes (in MWh, MW-Month, MMBtu) | MWh	(21,987,241)	[1]
Power | Sales | 2017
Derivative [Line Items]
Volumes (in MWh, MW-Month, MMBtu) | MWh	(24,721,341)
Power | Sales | 2018
Derivative [Line Items]
Volumes (in MWh, MW-Month, MMBtu) | MWh	(7,568,633)
Power | Sales | Thereafter
Derivative [Line Items]
Volumes (in MWh, MW-Month, MMBtu) | MWh	(719,594)
Capacity | Sales | 2016
Derivative [Line Items]
Volumes (in MWh, MW-Month, MMBtu) | MW-M	(3,050)	[1]
Capacity | Sales | 2017
Derivative [Line Items]
Volumes (in MWh, MW-Month, MMBtu) | MW-M	(3,750)
Capacity | Sales | 2018
Derivative [Line Items]
Volumes (in MWh, MW-Month, MMBtu) | MW-M	(3,746)
Capacity | Sales | Thereafter
Derivative [Line Items]
Volumes (in MWh, MW-Month, MMBtu) | MW-M	(1,559)
Gas | Purchases | 2016
Derivative [Line Items]
Volumes (in MWh, MW-Month, MMBtu) | MMBTU	(87,035,186)	[1]
Gas | Purchases | 2017
Derivative [Line Items]
Volumes (in MWh, MW-Month, MMBtu) | MMBTU	(29,084,926)
Gas | Purchases | 2018
Derivative [Line Items]
Volumes (in MWh, MW-Month, MMBtu) | MMBTU	(16,333,048)
Gas | Purchases | Thereafter
Derivative [Line Items]
Volumes (in MWh, MW-Month, MMBtu) | MMBTU	(3,206,105)
FTRs | Purchases | 2016
Derivative [Line Items]
Volumes (in MWh, MW-Month, MMBtu) | MW-M	(7,212)	[1]
FTRs | Purchases | 2017
Derivative [Line Items]
Volumes (in MWh, MW-Month, MMBtu) | MW-M	(2,929)
FTRs | Purchases | 2018
Derivative [Line Items]
Volumes (in MWh, MW-Month, MMBtu) | MW-M	0
FTRs | Purchases | Thereafter
Derivative [Line Items]
Volumes (in MWh, MW-Month, MMBtu) | MW-M	0
Oil | Purchases | 2016
Derivative [Line Items]
Volumes (in barrels) | bbl	33,655	[1]
Oil | Purchases | 2017
Derivative [Line Items]
Volumes (in barrels) | bbl	0
Oil | Purchases | 2018
Derivative [Line Items]
Volumes (in barrels) | bbl	0
Oil | Purchases | Thereafter
Derivative [Line Items]
Volumes (in barrels) | bbl	0
CRR purchase contracts | Purchases | 2016
Derivative [Line Items]
Volumes (in MWh, MW-Month, MMBtu) | MWh	(1,138,739)	[1]
CRR purchase contracts | Purchases | 2017
Derivative [Line Items]
Volumes (in MWh, MW-Month, MMBtu) | MWh	(641,989)
CRR purchase contracts | Purchases | 2018
Derivative [Line Items]
Volumes (in MWh, MW-Month, MMBtu) | MWh	0
CRR purchase contracts | Purchases | Thereafter
Derivative [Line Items]
Volumes (in MWh, MW-Month, MMBtu) | MWh	0
Emission Allowances | Purchases | 2016
Derivative [Line Items]
Volumes (tons) | T	(3,322,531)	[1]
Emission Allowances | Purchases | 2017
Derivative [Line Items]
Volumes (tons) | T	(97,548)
Emission Allowances | Purchases | 2018
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.