Derivative Assets And Liabilities Table - Outstanding Commodity-Related Derivatives (Details)		3 Months Ended	12 Months Ended
		Mar. 31, 2018 MB_bls BBtu Megawatt bushels	Dec. 31, 2017 barrels BBtu Megawatt bbl bushels
Mark-To-Market Derivatives [Member] | Maximum [Member] | Non Trading [Member] | Forward Physical Contracts [Member]
Maximum Term Of Commodity Derivatives		2020	2020
Mark-To-Market Derivatives [Member] | Maximum [Member] | Non Trading [Member] | Forwards Swaps [Member]
Maximum Term Of Commodity Derivatives			2019
Mark-To-Market Derivatives [Member] | Maximum [Member] | Non Trading [Member] | Future [Member]
Maximum Term Of Commodity Derivatives			2019
Mark-To-Market Derivatives [Member] | Maximum [Member] | Trading [Member] | Basis Swaps IFERC/NYMEX [Member]
Maximum Term Of Commodity Derivatives			2020
Mark-To-Market Derivatives [Member] | Minimum [Member] | Non Trading [Member] | Forward Physical Contracts [Member]
Maximum Term Of Commodity Derivatives		2018	2018
Mark-To-Market Derivatives [Member] | Minimum [Member] | Non Trading [Member] | Forwards Swaps [Member]
Maximum Term Of Commodity Derivatives			2018
Mark-To-Market Derivatives [Member] | Minimum [Member] | Non Trading [Member] | Future [Member]
Maximum Term Of Commodity Derivatives			2018
Mark-To-Market Derivatives [Member] | Minimum [Member] | Trading [Member] | Basis Swaps IFERC/NYMEX [Member]
Maximum Term Of Commodity Derivatives			2018
Natural Gas [Member] | Mark-To-Market Derivatives [Member] | Non Trading [Member] | Basis Swaps IFERC/NYMEX [Member] | Long [Member]
Notional Volume		(9,750)	(4,650)
Natural Gas [Member] | Mark-To-Market Derivatives [Member] | Non Trading [Member] | Swing Swaps IFERC [Member] | Short [Member]
Notional Volume		(24,825)
Natural Gas [Member] | Mark-To-Market Derivatives [Member] | Non Trading [Member] | Swing Swaps IFERC [Member] | Long [Member]
Notional Volume			(87,253)
Natural Gas [Member] | Mark-To-Market Derivatives [Member] | Non Trading [Member] | Fixed Swaps/Futures [Member] | Short [Member]
Notional Volume		(4,540)	(4,390)
Natural Gas [Member] | Mark-To-Market Derivatives [Member] | Non Trading [Member] | Forward Physical Contracts [Member] | Short [Member]
Notional Volume		(224,178)	(145,105)
Natural Gas [Member] | Mark-To-Market Derivatives [Member] | Trading [Member] | Basis Swaps IFERC/NYMEX [Member] | Long [Member]
Notional Volume	[1]	(82,493)	(48,510)
Natural Gas [Member] | Mark-To-Market Derivatives [Member] | Trading [Member] | Fixed Swaps/Futures [Member] | Short [Member]
Maximum Term Of Commodity Derivatives			2018
Natural Gas [Member] | Mark-To-Market Derivatives [Member] | Trading [Member] | Fixed Swaps/Futures [Member] | Long [Member]
Notional Volume		(1,008)	(1,078)
Maximum Term Of Commodity Derivatives		2018
Natural Gas [Member] | Mark-To-Market Derivatives [Member] | Trading [Member] | Options - Puts [Member]
Maximum Term Of Commodity Derivatives		2018
Natural Gas [Member] | Mark-To-Market Derivatives [Member] | Trading [Member] | Options - Puts [Member] | Long [Member]
Notional Volume		(13,000)	(13,000)
Maximum Term Of Commodity Derivatives			2018
Natural Gas [Member] | Mark-To-Market Derivatives [Member] | Trading [Member] | Options - Calls [Member]
Maximum Term Of Commodity Derivatives		2018
Natural Gas [Member] | Mark-To-Market Derivatives [Member] | Trading [Member] | Options - Calls [Member] | Long [Member]
Notional Volume		(460)
Natural Gas [Member] | Mark-To-Market Derivatives [Member] | Maximum [Member] | Non Trading [Member] | Basis Swaps IFERC/NYMEX [Member]
Maximum Term Of Commodity Derivatives		2020	2020
Natural Gas [Member] | Mark-To-Market Derivatives [Member] | Maximum [Member] | Non Trading [Member] | Swing Swaps IFERC [Member]
Maximum Term Of Commodity Derivatives		2019	2019
Natural Gas [Member] | Mark-To-Market Derivatives [Member] | Maximum [Member] | Non Trading [Member] | Fixed Swaps/Futures [Member]
Maximum Term Of Commodity Derivatives		2019	2019
Natural Gas [Member] | Mark-To-Market Derivatives [Member] | Maximum [Member] | Trading [Member] | Basis Swaps IFERC/NYMEX [Member]
Maximum Term Of Commodity Derivatives		2020
Natural Gas [Member] | Mark-To-Market Derivatives [Member] | Minimum [Member] | Non Trading [Member] | Basis Swaps IFERC/NYMEX [Member]
Maximum Term Of Commodity Derivatives		2018	2018
Natural Gas [Member] | Mark-To-Market Derivatives [Member] | Minimum [Member] | Non Trading [Member] | Swing Swaps IFERC [Member]
Maximum Term Of Commodity Derivatives		2018	2018
Natural Gas [Member] | Mark-To-Market Derivatives [Member] | Minimum [Member] | Non Trading [Member] | Fixed Swaps/Futures [Member]
Maximum Term Of Commodity Derivatives		2018	2018
Natural Gas [Member] | Mark-To-Market Derivatives [Member] | Minimum [Member] | Trading [Member] | Basis Swaps IFERC/NYMEX [Member]
Maximum Term Of Commodity Derivatives		2018
Natural Gas [Member] | Fair Value Hedging [Member] | Non Trading [Member] | Basis Swaps IFERC/NYMEX [Member] | Short [Member]
Notional Volume		(18,685)	(39,770)
Maximum Term Of Commodity Derivatives		2018	2018
Natural Gas [Member] | Fair Value Hedging [Member] | Non Trading [Member] | Fixed Swaps/Futures [Member] | Short [Member]
Notional Volume		(18,685)	(39,770)
Maximum Term Of Commodity Derivatives		2018	2018
Natural Gas [Member] | Fair Value Hedging [Member] | Non Trading [Member] | Hedged Item - Inventory (MMBtu) [Member] | Long [Member]
Notional Volume		(18,685)	(39,770)
Maximum Term Of Commodity Derivatives		2018	2018
Power [Member] | Mark-To-Market Derivatives [Member] | Trading [Member] | Options - Puts [Member]
Maximum Term Of Commodity Derivatives		2018
Power [Member] | Mark-To-Market Derivatives [Member] | Trading [Member] | Options - Puts [Member] | Short [Member]
Notional Volume | Megawatt			(153,600)
Maximum Term Of Commodity Derivatives			2018
Power [Member] | Mark-To-Market Derivatives [Member] | Trading [Member] | Options - Puts [Member] | Long [Member]
Notional Volume | Megawatt		(238,400)
Power [Member] | Mark-To-Market Derivatives [Member] | Trading [Member] | Call Option [Member] | Long [Member]
Notional Volume | Megawatt		(349,600)	(137,600)
Maximum Term Of Commodity Derivatives		2018	2018
Power [Member] | Mark-To-Market Derivatives [Member] | Trading [Member] | Forwards Swaps [Member] | Long [Member]
Notional Volume | Megawatt		(236,680)	(435,960)
Power [Member] | Mark-To-Market Derivatives [Member] | Trading [Member] | Future [Member]
Maximum Term Of Commodity Derivatives		2018	2018
Power [Member] | Mark-To-Market Derivatives [Member] | Trading [Member] | Future [Member] | Short [Member]
Notional Volume | Megawatt			(25,760)
Power [Member] | Mark-To-Market Derivatives [Member] | Trading [Member] | Future [Member] | Long [Member]
Notional Volume | Megawatt		(126,200)
Power [Member] | Mark-To-Market Derivatives [Member] | Maximum [Member] | Trading [Member] | Forwards Swaps [Member]
Maximum Term Of Commodity Derivatives		2019	2019
Power [Member] | Mark-To-Market Derivatives [Member] | Maximum [Member] | Trading [Member] | Future [Member]
Maximum Term Of Commodity Derivatives
Power [Member] | Mark-To-Market Derivatives [Member] | Minimum [Member] | Trading [Member] | Forwards Swaps [Member]
Maximum Term Of Commodity Derivatives		2018	2018
Power [Member] | Mark-To-Market Derivatives [Member] | Minimum [Member] | Trading [Member] | Future [Member]
Maximum Term Of Commodity Derivatives
Natural Gas Liquids and Crude [Member] | Mark-To-Market Derivatives [Member] | Non Trading [Member] | Forwards Swaps [Member] | Long [Member]
Notional Volume		(38,874)	(6,744)
Natural Gas Liquids and Crude [Member] | Mark-To-Market Derivatives [Member] | Maximum [Member] | Non Trading [Member] | Forwards Swaps [Member]
Maximum Term Of Commodity Derivatives		2019
Natural Gas Liquids and Crude [Member] | Mark-To-Market Derivatives [Member] | Minimum [Member] | Non Trading [Member] | Forwards Swaps [Member]
Maximum Term Of Commodity Derivatives		2018
Refined Products [Member] | Mark-To-Market Derivatives [Member] | Non Trading [Member] | Future [Member] | Short [Member]
Notional Volume		(871)	(3,901)
Refined Products [Member] | Mark-To-Market Derivatives [Member] | Maximum [Member] | Non Trading [Member] | Future [Member]
Maximum Term Of Commodity Derivatives		2019
Refined Products [Member] | Mark-To-Market Derivatives [Member] | Minimum [Member] | Non Trading [Member] | Future [Member]
Maximum Term Of Commodity Derivatives		2018
Corn [Member] | Mark-To-Market Derivatives [Member] | Non Trading [Member] | Future [Member]
Maximum Term Of Commodity Derivatives		2018	2018
Corn [Member] | Mark-To-Market Derivatives [Member] | Non Trading [Member] | Future [Member] | Short [Member]
Notional Volume | bushels		(780,000)
Corn [Member] | Mark-To-Market Derivatives [Member] | Non Trading [Member] | Future [Member] | Long [Member]
Notional Volume | bushels			(1,870,000)
Corn [Member] | Mark-To-Market Derivatives [Member] | Maximum [Member] | Non Trading [Member] | Future [Member]
Maximum Term Of Commodity Derivatives
Corn [Member] | Mark-To-Market Derivatives [Member] | Minimum [Member] | Non Trading [Member] | Future [Member]
Maximum Term Of Commodity Derivatives

[1] Includes aggregate amounts for open positions related to Houston Ship Channel, Waha Hub, NGPL TexOk, West Louisiana Zone and Henry Hub locations.