Derivative Accounting - Schedule of Gross Notional Amounts Outstanding (Details) - Commodity Contracts	6 Months Ended
Jun. 30, 2017 GWh Bcf
Outstanding gross notional amount of derivatives
Power \| GWh	908
Gas \| Bcf	247

Nominal energy measure used to calculate payments on a derivative instrument.

No definition available.

Name:	us-gaap_DerivativeNonmonetaryNotionalAmountEnergyMeasure
Namespace Prefix:	us-gaap_
Data Type:	num:energyItemType
Balance Type:	na
Period Type:	duration