Derivatives and Risk Management (Schedule of Derivative Instruments Notional Amount, Weighted Average Contract Prices and Fair Value) (Details) - Not Designated as Hedging Instrument [Member] ft³ in Billions	9 Months Ended
	Sep. 30, 2018 USD ($) $ / bbl $ / MMBTU MBbls ft³
Sold Fixed Price Swaps -2018 [Member] | Natural Gas [Member]
Derivative [Line Items]
Volume | ft³	42.0
Average price | $ / MMBTU	2.91
Fair Value	$ (5,000,000)
Sold Fixed Price Swaps -2018 [Member] | Propane [Member]
Derivative [Line Items]
Volume | MBbls	778
Average price | $ / bbl	34.86
Fair Value	$ (8,000,000)
Sold Fixed Price Swaps -2018 [Member] | Ethane [Member]
Derivative [Line Items]
Volume | MBbls	1,389
Average price | $ / bbl	13.30
Fair Value	$ (13,000,000)
Sold Fixed Price Swaps -2019 [Member] | Natural Gas [Member]
Derivative [Line Items]
Volume | ft³	156.0
Average price | $ / MMBTU	2.92
Fair Value	$ 29,000,000
Sold Fixed Price Swaps -2019 [Member] | Propane [Member]
Derivative [Line Items]
Volume | MBbls	1,506
Average price | $ / bbl	32.63
Fair Value	$ (11,000,000)
Sold Fixed Price Swaps -2019 [Member] | Ethane [Member]
Derivative [Line Items]
Volume | MBbls	3,322
Average price | $ / bbl	13.38
Fair Value	$ (16,000,000)
Sold Fixed Price Swaps -2020 [Member] | Natural Gas [Member]
Derivative [Line Items]
Volume | ft³	2.0
Average price | $ / MMBTU	2.77
Storage Purchased Fixed Price Swaps - 2018 [Member]
Derivative [Line Items]
Volume | ft³	0.2
Average price | $ / MMBTU	2.77
Storage Sold Fixed Price Swaps - 2018 [Member]
Derivative [Line Items]
Volume | ft³	0.2
Average price | $ / MMBTU	2.89
Storage Sold Fixed Price Swaps - 2019 [Member]
Derivative [Line Items]
Volume | ft³	0.8
Average price | $ / MMBTU	3.03
Two-way Costless-collars - 2018 [Member] | Natural Gas [Member]
Derivative [Line Items]
Volume | ft³	6.0
Two-way Costless-collars - 2018 Purchased Puts [Member] | Natural Gas [Member]
Derivative [Line Items]
Floor price per MMBtu | $ / MMBTU	2.90
Two-way Costless-collars - 2018 Sold Calls [Member] | Natural Gas [Member]
Derivative [Line Items]
Cap price per MMBtu | $ / MMBTU	3.27
Two-way Costless-collars - 2019 [Member] | Natural Gas [Member]
Derivative [Line Items]
Volume | ft³	53.0
Fair Value	$ 8,000,000
Two-way Costless-collars - 2019 Purchased Puts [Member] | Natural Gas [Member]
Derivative [Line Items]
Floor price per MMBtu | $ / MMBTU	2.80
Two-way Costless-collars - 2019 Sold Calls [Member] | Natural Gas [Member]
Derivative [Line Items]
Cap price per MMBtu | $ / MMBTU	2.98
Three-way Costless-collars - 2018 [Member] | Natural Gas [Member]
Derivative [Line Items]
Volume | ft³	72.0
Fair Value	$ 3,000,000
Three-way Costless-collars - 2018 Sold Puts [Member] | Natural Gas [Member]
Derivative [Line Items]
Floor price per MMBtu | $ / MMBTU	2.40
Three-way Costless-collars - 2018 Purchased Puts [Member] | Natural Gas [Member]
Derivative [Line Items]
Floor price per MMBtu | $ / MMBTU	2.97
Three-way Costless-collars - 2018 Sold Calls [Member] | Natural Gas [Member]
Derivative [Line Items]
Cap price per MMBtu | $ / MMBTU	3.37
Three-Way Costless Collars - 2019 [Member] | Natural Gas [Member]
Derivative [Line Items]
Volume | ft³	163.0
Fair Value	$ 13,000,000
Three-way Costless-collars - 2019 Sold Puts [Member] | Natural Gas [Member]
Derivative [Line Items]
Floor price per MMBtu | $ / MMBTU	2.47
Three-way Costless-collars - 2019 Purchased Puts [Member] | Natural Gas [Member]
Derivative [Line Items]
Floor price per MMBtu | $ / MMBTU	2.89
Three-way Costless-collars - 2019 Sold Calls [Member] | Natural Gas [Member]
Derivative [Line Items]
Cap price per MMBtu | $ / MMBTU	3.26
Three-way Costless-collars - 2020 [Member] | Natural Gas [Member]
Derivative [Line Items]
Volume | ft³	47.0
Fair Value	$ 3,000,000
Three-Way Costless Collars - 2020 Sold Puts [Member] | Natural Gas [Member]
Derivative [Line Items]
Floor price per MMBtu | $ / MMBTU	2.43
Three-Way Costless Collars - 2020 Purchased Puts [Member] | Natural Gas [Member]
Derivative [Line Items]
Floor price per MMBtu | $ / MMBTU	2.80
Three-Way Costless Collars - 2020 Sold Calls [Member] | Natural Gas [Member]
Derivative [Line Items]
Cap price per MMBtu | $ / MMBTU	3.09
Financial protection on production - 2018 [Member] | Natural Gas [Member]
Derivative [Line Items]
Volume | ft³	120.0
Fair Value	$ (2,000,000)
Financial protection on production - 2019 [Member] | Natural Gas [Member]
Derivative [Line Items]
Volume | ft³	372.0
Fair Value	$ 50,000,000
Financial protection on production - 2020 [Member] | Natural Gas [Member]
Derivative [Line Items]
Volume | ft³	49.0
Fair Value	$ 3,000,000
Financial protection on storage - 2018 [Member]
Derivative [Line Items]
Volume | ft³	0.6
Financial protection on storage - 2019 [Member]
Derivative [Line Items]
Volume | ft³	1.5
Sold Basis Swaps - 2018 [Member] | Natural Gas [Member]
Derivative [Line Items]
Volume | ft³	16.0
Basis differential per MMBtu	$ (0.55)
Fair Value	$ (2,000,000)
Sold Basis Swaps - 2019 [Member] | Natural Gas [Member]
Derivative [Line Items]
Volume | ft³	32.0
Basis differential per MMBtu	$ 0.23
Fair Value	$ (8,000,000)
Sold Basis Swaps [Member] | Natural Gas [Member]
Derivative [Line Items]
Volume | ft³	48.0
Fair Value	$ (10,000,000)
Storage Purchased Basis Swaps - 2018 [Member]
Derivative [Line Items]
Volume | ft³	0.1
Basis differential per MMBtu	$ (0.89)
Storage Sold Basis Swaps - 2018 [Member]
Derivative [Line Items]
Volume | ft³	0.1
Basis differential per MMBtu	$ (0.62)
Storage Sold Basis Swaps - 2019 [Member]
Derivative [Line Items]
Volume | ft³	0.7
Basis differential per MMBtu	$ (0.44)
Purchased Call Options - 2020 [Member] | Natural Gas [Member]
Derivative [Line Items]
Volume | ft³	68.0
Cap price per MMBtu | $ / MMBTU	3.63
Fair Value	$ 2,000,000
Purchased Call Options - 2021 [Member] | Natural Gas [Member]
Derivative [Line Items]
Volume | ft³	57.0
Cap price per MMBtu | $ / MMBTU	3.52
Fair Value	$ 2,000,000
Purchased Call Options [Member] | Natural Gas [Member]
Derivative [Line Items]
Volume | ft³	125.0
Fair Value	$ 4,000,000
Sold Call Options - 2018 [Member] | Natural Gas [Member]
Derivative [Line Items]
Volume | ft³	16.0
Cap price per MMBtu | $ / MMBTU	3.50
Sold Call Options - 2018 [Member] | Oil [Member]
Derivative [Line Items]
Volume | MBbls	276
Cap price per MMBtu | $ / bbl	65.00
Fair Value	$ (2,000,000)
Sold Call Options - 2019 [Member] | Natural Gas [Member]
Derivative [Line Items]
Volume | ft³	52.0
Cap price per MMBtu | $ / MMBTU	3.50
Fair Value	$ (2,000,000)
Sold Call Options - 2019 [Member] | Oil [Member]
Derivative [Line Items]
Volume | MBbls	270
Cap price per MMBtu | $ / bbl	65.00
Fair Value	$ (3,000,000)
Sold Call Options - 2020 [Member] | Natural Gas [Member]
Derivative [Line Items]
Volume | ft³	136.0
Cap price per MMBtu | $ / MMBTU	3.39
Fair Value	$ (8,000,000)
Sold Call Options - 2021 [Member] | Natural Gas [Member]
Derivative [Line Items]
Volume | ft³	114.0
Cap price per MMBtu | $ / MMBTU	3.33
Fair Value	$ (6,000,000)
Sold Call Options [Member] | Natural Gas [Member]
Derivative [Line Items]
Volume | ft³	318.0
Fair Value	$ (16,000,000)
Sold Call Options [Member] | Oil [Member]
Derivative [Line Items]
Volume | MBbls	546
Fair Value	$ (5,000,000)