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Risk Management Activities - Commodity Risk Management (Details) - Commodity risk management
t in Thousands, gal in Thousands, kJ in Billions
Dec. 31, 2017
MWh
kJ
t
gal
Dec. 31, 2016
MWh
kJ
t
gal
Electricity [Member] | Cash flow hedges    
Disclosure of detailed information about hedges [line items]    
Notional amount sold 1,997,000 4,916,000
Notional amount purchased 44,000 0
Electricity [Member] | Non-Hedges [Member]    
Disclosure of detailed information about hedges [line items]    
Notional amount sold 14,688,000 19,362,000
Notional amount purchased 7,348,000 19,060,000
Natural gas | Non-Hedges [Member]    
Disclosure of detailed information about hedges [line items]    
Notional amount sold | kJ 74,194 146,113
Notional amount purchased | kJ 103,805 173,187
Transmission | Non-Hedges [Member]    
Disclosure of detailed information about hedges [line items]    
Notional amount sold 1,000 0
Notional amount purchased 3,455,000 3,429,000
Emissions | Non-Hedges [Member]    
Disclosure of detailed information about hedges [line items]    
Derivative, Nonmonetary Notional Amount Sold, Emissions | t 516 1,370
Derivative, Nonmonetary Notional Amount Purchased, Emissions | t 717 1,370
Heating Oil | Non-Hedges [Member]    
Disclosure of detailed information about hedges [line items]    
Derivative, Nonmonetary Notional Amount Sold In Gallons | gal 0 0
Derivative, Nonmonetary Notional Amount Purchased In Gallons | gal 0 294