Derivative Instruments (Details) $ in Millions |
6 Months Ended | 12 Months Ended |
|---|---|---|
|
Jun. 30, 2015
USD ($)
MWh
MMBTU
t
|
Dec. 31, 2014
USD ($)
MWh
MMBTU
t
|
|
| Power [Member] | ||
| Derivative [Line Items] | ||
| Derivative, Nonmonetary Notional Amount, Energy Measure | (99) | (62) |
| Natural Gas [Member] | ||
| Derivative [Line Items] | ||
| Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU | 874 | 291 |
| Environmental Credits [Member] | ||
| Derivative [Line Items] | ||
| Derivative, Nonmonetary Notional Amount, Mass | t | 3 | 0 |
| Interest Rate Swap [Member] | ||
| Derivative [Line Items] | ||
| Derivative, Notional Amount | $ | $ 1,411 | $ 1,431 |