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Derivative Assets And Liabilities (Outstanding Commodity-Related Derivatives) (Details)
MB_bls in Thousands, BBtu in Thousands
 12 Months Ended
Dec. 31, 2017
bushels
MW
BBtu
MB_bls
Dec. 31, 2016
bushels
MW
BBtu
MB_bls
Natural Gas [Member] | Mark-To-Market Derivatives [Member] | Non Trading [Member] | Swing Swaps IFERC [Member]    
Term Of Commodity Derivatives   2017
Natural Gas [Member] | Mark-To-Market Derivatives [Member] | Non Trading [Member] | Swing Swaps IFERC [Member] | Maximum [Member]    
Term Of Commodity Derivatives 2019  
Natural Gas [Member] | Mark-To-Market Derivatives [Member] | Non Trading [Member] | Swing Swaps IFERC [Member] | Minimum [Member]    
Term Of Commodity Derivatives 2018  
Natural Gas [Member] | Mark-To-Market Derivatives [Member] | Non Trading [Member] | Fixed Swaps/Futures [Member] | Maximum [Member]    
Term Of Commodity Derivatives 2019 2018
Natural Gas [Member] | Mark-To-Market Derivatives [Member] | Non Trading [Member] | Fixed Swaps/Futures [Member] | Minimum [Member]    
Term Of Commodity Derivatives 2018 2017
Natural Gas [Member] | Mark-To-Market Derivatives [Member] | Non Trading [Member] | Basis Swaps IFERC NYMEX [Member] | Maximum [Member]    
Term Of Commodity Derivatives 2020 2018
Natural Gas [Member] | Mark-To-Market Derivatives [Member] | Non Trading [Member] | Basis Swaps IFERC NYMEX [Member] | Minimum [Member]    
Term Of Commodity Derivatives 2018 2017
Natural Gas [Member] | Mark-To-Market Derivatives [Member] | Non Trading [Member] | Forward Physical Contracts [Member]    
Term Of Commodity Derivatives   2017
Natural Gas [Member] | Mark-To-Market Derivatives [Member] | Non Trading [Member] | Forward Physical Contracts [Member] | Maximum [Member]    
Term Of Commodity Derivatives 2020  
Natural Gas [Member] | Mark-To-Market Derivatives [Member] | Non Trading [Member] | Forward Physical Contracts [Member] | Minimum [Member]    
Term Of Commodity Derivatives 2018  
Natural Gas [Member] | Mark-To-Market Derivatives [Member] | Trading [Member] | Fixed Swaps/Futures [Member]    
Term Of Commodity Derivatives 2018 2017
Natural Gas [Member] | Mark-To-Market Derivatives [Member] | Trading [Member] | Basis Swaps IFERC NYMEX [Member]    
Term Of Commodity Derivatives   2017
Natural Gas [Member] | Mark-To-Market Derivatives [Member] | Trading [Member] | Basis Swaps IFERC NYMEX [Member] | Maximum [Member]    
Term Of Commodity Derivatives 2020  
Natural Gas [Member] | Mark-To-Market Derivatives [Member] | Trading [Member] | Basis Swaps IFERC NYMEX [Member] | Minimum [Member]    
Term Of Commodity Derivatives 2018  
Natural Gas [Member] | Mark-To-Market Derivatives [Member] | Trading [Member] | Options - Calls [Member]    
Term Of Commodity Derivatives 2018  
Natural Gas [Member] | Fair Value Hedging [Member] | Non Trading [Member] | Hedged Item - Inventory (MMBtu) [Member]    
Term Of Commodity Derivatives 2018 2017
Natural Gas [Member] | Fair Value Hedging [Member] | Non Trading [Member] | Fixed Swaps/Futures [Member]    
Term Of Commodity Derivatives 2018 2017
Natural Gas [Member] | Fair Value Hedging [Member] | Non Trading [Member] | Basis Swaps IFERC NYMEX [Member]    
Term Of Commodity Derivatives 2018 2017
Natural Gas [Member] | Short [Member] | Mark-To-Market Derivatives [Member] | Non Trading [Member] | Swing Swaps IFERC [Member]    
Derivative, Nonmonetary Notional Amount   5,663
Natural Gas [Member] | Short [Member] | Mark-To-Market Derivatives [Member] | Non Trading [Member] | Fixed Swaps/Futures [Member]    
Derivative, Nonmonetary Notional Amount 4,390 52,653
Natural Gas [Member] | Short [Member] | Mark-To-Market Derivatives [Member] | Non Trading [Member] | Forward Physical Contracts [Member]    
Derivative, Nonmonetary Notional Amount 145,105 22,492
Natural Gas [Member] | Short [Member] | Mark-To-Market Derivatives [Member] | Trading [Member] | Fixed Swaps/Futures [Member]    
Derivative, Nonmonetary Notional Amount   683
Natural Gas [Member] | Short [Member] | Fair Value Hedging [Member] | Non Trading [Member] | Fixed Swaps/Futures [Member]    
Derivative, Nonmonetary Notional Amount 39,770 36,370
Natural Gas [Member] | Short [Member] | Fair Value Hedging [Member] | Non Trading [Member] | Basis Swaps IFERC NYMEX [Member]    
Derivative, Nonmonetary Notional Amount 39,770 36,370
Natural Gas [Member] | Long [Member] | Mark-To-Market Derivatives [Member] | Non Trading [Member] | Swing Swaps IFERC [Member]    
Derivative, Nonmonetary Notional Amount 87,253  
Natural Gas [Member] | Long [Member] | Mark-To-Market Derivatives [Member] | Non Trading [Member] | Basis Swaps IFERC NYMEX [Member]    
Derivative, Nonmonetary Notional Amount 4,650 10,750
Natural Gas [Member] | Long [Member] | Mark-To-Market Derivatives [Member] | Trading [Member] | Fixed Swaps/Futures [Member]    
Derivative, Nonmonetary Notional Amount 1,078  
Natural Gas [Member] | Long [Member] | Mark-To-Market Derivatives [Member] | Trading [Member] | Basis Swaps IFERC NYMEX [Member]    
Derivative, Nonmonetary Notional Amount 48,510 2,243
Natural Gas [Member] | Long [Member] | Mark-To-Market Derivatives [Member] | Trading [Member] | Options - Calls [Member]    
Derivative, Nonmonetary Notional Amount 13,000 0
Natural Gas [Member] | Long [Member] | Fair Value Hedging [Member] | Non Trading [Member] | Hedged Item - Inventory (MMBtu) [Member]    
Derivative, Nonmonetary Notional Amount 39,770 36,370
Power [Member] | Mark-To-Market Derivatives [Member] | Trading [Member] | Options - Calls [Member]    
Term Of Commodity Derivatives 2018 2017
Power [Member] | Mark-To-Market Derivatives [Member] | Trading [Member] | Forward Swaps [Member] | Maximum [Member]    
Term Of Commodity Derivatives 2019 2018
Power [Member] | Mark-To-Market Derivatives [Member] | Trading [Member] | Forward Swaps [Member] | Minimum [Member]    
Term Of Commodity Derivatives 2018 2017
Power [Member] | Mark-To-Market Derivatives [Member] | Trading [Member] | Future [Member]    
Term Of Commodity Derivatives 2018  
Power [Member] | Mark-To-Market Derivatives [Member] | Trading [Member] | Future [Member] | Maximum [Member]    
Term Of Commodity Derivatives   2018
Power [Member] | Mark-To-Market Derivatives [Member] | Trading [Member] | Future [Member] | Minimum [Member]    
Term Of Commodity Derivatives   2017
Power [Member] | Mark-To-Market Derivatives [Member] | Trading [Member] | Put Option [Member]    
Term Of Commodity Derivatives 2018 2017
Power [Member] | Short [Member] | Mark-To-Market Derivatives [Member] | Trading [Member] | Future [Member]    
Derivative, Nonmonetary Notional Amount | MW 25,760  
Power [Member] | Short [Member] | Mark-To-Market Derivatives [Member] | Trading [Member] | Put Option [Member]    
Derivative, Nonmonetary Notional Amount | MW 153,600 50,400
Power [Member] | Long [Member] | Mark-To-Market Derivatives [Member] | Trading [Member] | Options - Calls [Member]    
Derivative, Nonmonetary Notional Amount | MW 137,600 186,400
Power [Member] | Long [Member] | Mark-To-Market Derivatives [Member] | Trading [Member] | Forward Swaps [Member]    
Derivative, Nonmonetary Notional Amount | MW 435,960 391,880
Power [Member] | Long [Member] | Mark-To-Market Derivatives [Member] | Trading [Member] | Future [Member]    
Derivative, Nonmonetary Notional Amount | MW   109,564
WTI Crude Oil [Member] | Mark-To-Market Derivatives [Member] | Trading [Member] | Future [Member]    
Term Of Commodity Derivatives   2017
WTI Crude Oil [Member] | Short [Member] | Mark-To-Market Derivatives [Member] | Trading [Member] | Future [Member]    
Derivative, Nonmonetary Notional Amount | MB_bls 0 617
Natural Gas Liquids [Member] | Mark-To-Market Derivatives [Member] | Non Trading [Member] | Forward Swaps [Member]    
Term Of Commodity Derivatives   2017
Natural Gas Liquids [Member] | Mark-To-Market Derivatives [Member] | Non Trading [Member] | Forward Swaps [Member] | Maximum [Member]    
Term Of Commodity Derivatives 2019  
Natural Gas Liquids [Member] | Mark-To-Market Derivatives [Member] | Non Trading [Member] | Forward Swaps [Member] | Minimum [Member]    
Term Of Commodity Derivatives 2018  
Natural Gas Liquids [Member] | Short [Member] | Mark-To-Market Derivatives [Member] | Non Trading [Member] | Forward Swaps [Member]    
Derivative, Nonmonetary Notional Amount | MB_bls   5,787
Natural Gas Liquids [Member] | Long [Member] | Mark-To-Market Derivatives [Member] | Non Trading [Member] | Forward Swaps [Member]    
Derivative, Nonmonetary Notional Amount | MB_bls 6,744  
Refined Products [Member] | Mark-To-Market Derivatives [Member] | Non Trading [Member] | Future [Member]    
Term Of Commodity Derivatives   2017
Refined Products [Member] | Mark-To-Market Derivatives [Member] | Non Trading [Member] | Future [Member] | Maximum [Member]    
Term Of Commodity Derivatives 2019  
Refined Products [Member] | Mark-To-Market Derivatives [Member] | Non Trading [Member] | Future [Member] | Minimum [Member]    
Term Of Commodity Derivatives 2018  
Refined Products [Member] | Short [Member] | Mark-To-Market Derivatives [Member] | Non Trading [Member] | Future [Member]    
Derivative, Nonmonetary Notional Amount | MB_bls 3,901 3,144
Corn [Member] | Mark-To-Market Derivatives [Member] | Non Trading [Member] | Future [Member]    
Term Of Commodity Derivatives 2018 2017
Corn [Member] | Long [Member] | Mark-To-Market Derivatives [Member] | Non Trading [Member] | Future [Member]    
Derivative, Nonmonetary Notional Amount | bushels 1,870,000 1,580,000