Derivative Accounting - Outstanding Gross Notional Amounts Outstanding (Details)	12 Months Ended
Dec. 31, 2017 MWh Bcf	Dec. 31, 2016 MWh Bcf
Outstanding gross notional amount of derivatives
Power (in MWh) \| MWh	583,000	1,314,000
Gas (in bcf) \| Bcf	240	194

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