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Risk Management Activities - Commodity Risk Management (Details) - Commodity risk management
t in Thousands, kJ in Billions
Dec. 31, 2018
MWh
kJ
t
Dec. 31, 2017
MWh
kJ
t
Electricity | Cash flow hedges    
Disclosure of detailed information about hedges [line items]    
Notional amount sold 2,128,000 1,997,000
Notional amount purchased 0 44,000
Electricity | Non-Hedges    
Disclosure of detailed information about hedges [line items]    
Notional amount sold 58,885,000 14,688,000
Notional amount purchased 37,023,000 7,348,000
Natural gas | Non-Hedges    
Disclosure of detailed information about hedges [line items]    
Notional amount sold | kJ 80,413 74,194
Notional amount purchased | kJ 110,488 103,805
Transmission | Non-Hedges    
Disclosure of detailed information about hedges [line items]    
Notional amount sold 29,000 1,000
Notional amount purchased 11,163,000 3,455,000
Emissions | Non-Hedges    
Disclosure of detailed information about hedges [line items]    
Derivative, Nonmonetary Notional Amount Sold, Emissions | t 3,134 516
Derivative, Nonmonetary Notional Amount Purchased, Emissions | t 2,948 717