Derivative Accounting - Schedule of Gross Notional Amounts Outstanding (Details) - Commodity Contracts MWh in Thousands, MMcf in Thousands	3 Months Ended
Mar. 31, 2018 MWh MMcf	Mar. 31, 2017 MWh MMcf
Outstanding gross notional amount of derivatives
Power \| MWh	1,277	583
Gas \| MMcf	252	240

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