Derivative Financial Instruments (Textuals) (Details) $ in Millions	Dec. 31, 2016 USD ($) MMBTU $ / bbl $ / MMBTU MBbls
Derivative [Line Items]
Deferred Premium	$ 10
Percentage of contracted volumes held by the largest counterparty.	14.00%
Primary Derivative Counterparties	10
Total Derivative Counterparties	16
Percent of future hedged production with primary counterparties	85.00%
ConcentrationOfCounterpartiesUnderCreditFacility	84.00%
Natural gas contracts [Member]
Derivative [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Liability, at Fair Value	$ (54)
Oil contracts [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Total	$ 29
Year2018 [Member] | Natural gas contracts [Member] | Swap [Member]
Derivative [Line Items]
Derivative, Swap Type, Average Fixed Price | $ / MMBTU	3.01
Derivative Instruments Not Designated as Hedging Instruments, Liability, at Fair Value	$ (1)
Volume In Mmbtus | MMBTU	10,950,000
Year2018 [Member] | Natural gas contracts [Member] | Collars [Member]
Derivative [Line Items]
Derivative, Weighted Average Floor Price | $ / MMBTU	3
Derivative, Average Cap Price | $ / MMBTU	3.55
Derivative Instruments Not Designated as Hedging Instruments, Liability, at Fair Value	$ (2)
Volume In Mmbtus | MMBTU	18,150,000
Year2017 [Member] | Natural gas contracts [Member] | Swap [Member]
Derivative [Line Items]
Derivative, Swap Type, Average Fixed Price | $ / MMBTU	2.73
Derivative Instruments Not Designated as Hedging Instruments, Liability, at Fair Value	$ (24)
Volume In Mmbtus | MMBTU	27,375,000
Year2017 [Member] | Natural gas contracts [Member] | Collars [Member]
Derivative [Line Items]
Derivative, Weighted Average Floor Price | $ / MMBTU	2.82
Derivative, Average Cap Price | $ / MMBTU	3.23
Derivative Instruments Not Designated as Hedging Instruments, Liability, at Fair Value	$ (27)
Volume In Mmbtus | MMBTU	53,860,000
Year2017 [Member] | Oil contracts [Member] | Swap [Member]
Derivative [Line Items]
Volume in MBbls | MBbls	6,205
Derivative, Swap Type, Average Fixed Price | $ / bbl	45.430
Derivative Instruments Not Designated as Hedging Instruments, Liability, at Fair Value	$ (67)
Year2017 [Member] | Oil contracts [Member] | Fixed Price Swaps With Sold Puts [Member]
Derivative [Line Items]
Volume in MBbls | MBbls	4,468
Year2017 [Member] | Oil contracts [Member] | Fixed Price Swaps with Sold Puts - Swap [Member]
Derivative [Line Items]
Derivative, Swap Type, Average Fixed Price | $ / bbl	88.37
Derivative Asset, Fair Value, Total	$ 142
Year2017 [Member] | Oil contracts [Member] | Collars With Sold Puts [Member]
Derivative [Line Items]
Volume in MBbls | MBbls	2,080
Year2017 [Member] | Oil contracts [Member] | Fixed Price Swaps with Sold Puts - Put [Member]
Derivative [Line Items]
Derivative Average Additional Put Price | $ / bbl	73.28
Derivative Instruments Not Designated as Hedging Instruments, Liability, at Fair Value	$ (79)
Year2017 [Member] | Oil contracts [Member] | Collars with Sold Puts - Collar [Member]
Derivative [Line Items]
Derivative, Weighted Average Floor Price | $ / bbl	90
Derivative, Average Cap Price | $ / bbl	95.59
Derivative Asset, Fair Value, Total	$ 71
Year2017 [Member] | Oil contracts [Member] | Collars with Sold Puts - Sold Put [Member]
Derivative [Line Items]
Derivative Average Additional Put Price | $ / bbl	75
Derivative Instruments Not Designated as Hedging Instruments, Liability, at Fair Value	$ (41)
Year2017 [Member] | Oil contracts [Member] | Call Option [Member]
Derivative [Line Items]
Volume in MBbls | MBbls	6,548
Derivative, Average Cap Price | $ / bbl	73.81
Derivative Asset, Fair Value, Total	$ 3