Risk Management and Hedging Activities - MEC - Derivative Contract Volumes (Details) - Commodity derivative [Member] MWh in Millions, Dth in Millions	Jun. 30, 2017 Dth MWh	Dec. 31, 2016 Dth MWh
Electricity purchases (in megawatt hours) [Member]
Notional Amounts of Outstanding Derivative Positions [Line Items]
Derivative, nonmonetary notional amount \| MWh	11	5
Natural gas purchases (in decatherms) [Member]
Notional Amounts of Outstanding Derivative Positions [Line Items]
Derivative, nonmonetary notional amount \| Dth	279	271

Aggregate notional amount of derivative expressed in nonmonetary units. For example, the number of barrels specified in a fuel oil forward purchase contract.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Sentence Column B

Name:	invest_DerivativeNonmonetaryNotionalAmount
Namespace Prefix:	invest_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant