Organization and Summary of Significant Accounting Policies (Narrative) (Details)				1 Months Ended					3 Months Ended		9 Months Ended	12 Months Ended			144 Months Ended	180 Months Ended
	Dec. 03, 2018 USD ($)	Aug. 30, 2018 USD ($)	Jan. 12, 2018 shares	Dec. 31, 2018 USD ($) shares	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Jul. 31, 2016 USD ($) $ / shares shares	Jan. 31, 2015 shares	Dec. 31, 2018 USD ($) shares	Sep. 30, 2018 USD ($)	Sep. 30, 2017 shares	Dec. 31, 2018 USD ($) segment item $ / shares shares	Dec. 31, 2017 USD ($) item shares	Dec. 31, 2016 USD ($) item shares	Dec. 31, 2015 USD ($)	Dec. 31, 2018 USD ($) shares	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Nov. 30, 2015 USD ($)
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Number of segments | segment												2
Other income (loss), net													$ 5,000,000	$ (4,000,000)
Outstanding checks included in accounts payable				$ 34,000,000					$ 34,000,000			$ 34,000,000	17,000,000			$ 34,000,000
Net unevaluated costs excluded from amortization												$ 168,000,000	$ 138,000,000	$ 46,000,000	$ 1,403,000,000	$ 1,755,000,000
Period of time needed to calculate ceiling value of reserves												12 months	12 months	12 months
Number of derivative positions designated for hedge accounting | item												0	0	0
Impairments												$ 171,000,000		$ 2,321,000,000
Amortization of intangible asset												$ 9,000,000	$ 9,000,000	9,000,000
Natural gas, oil and NGL reserves discount												10.00%
Repayments of long-term debt												$ 2,095,000,000	$ 1,139,000,000	$ 1,175,000,000
Preferred stock, common shares issued as stock dividend | shares											10,040,306			9,917,799
Shares held in trust, shares | shares				10,653					10,653			10,653	31,269			10,653
Shares held in trust, shares released | shares												20,616
Treasury stock, shares | shares												39,061,269
Treasury stock acquired									$ 155,000,000	$ 25,000,000		$ 180,000,000
Treasury stock acquired, average cost per share | $ / shares												$ 4.63
Stock repurchase, authorized amount					$ 200,000,000					200,000,000
Sales Revenue, Net [Member] | Natural Gas, Oil and NGL [Member] | Customer Concentration Risk [Member]
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Concentration percentage												10.40%	10.30%
Accounting Standards Update 2017-07 [Member]
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Other income (loss), net														$ 5,000,000
Long-term Debt [Member] | 2015 Term Loan due December 2020 [Member]
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Repayments of long-term debt							$ 375,000,000
Debt instrument face amount																				$ 750,000,000
Senior Notes [Member]
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Repayments of long-term debt				$ 900,000,000			$ 700,000,000
Senior Notes [Member] | 2015 Term Loan due December 2020 [Member]
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Repayments of long-term debt						$ 327,000,000
Natural Gas [Member]
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Full cost ceiling test, price per MMBtu												$ 3.10	$ 2.98	2.48
Oil [Member]
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Full cost ceiling test, price per barrel												65.56	47.79	39.25
NGL [Member]
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Full cost ceiling test, price per barrel												$ 17.64	$ 14.41	6.74
Royal Dutch Shell Plc [Member]
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Number of subsidiaries of major customer with which business is conducted | item												2	2
United States and Canada [Member]
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Net book value that exceeds ceiling limitation																	$ 506,000,000	$ 297,000,000	$ 641,000,000
Midstream Services [Member]
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Other income (loss), net												$ (2,000,000)	$ 1,000,000	$ (2,000,000)
Impairments												155,000,000
Exploration and Production [Member]
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Other income (loss), net												2,000,000	$ 4,000,000
Impairments												15,000,000
Common Stock
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Shares issued | shares							98,900,000
Price per share | $ / shares							$ 13.00
Proceeds from issuance of shares							$ 1,247,000,000
Shares issued upon conversion | shares			74,998,614
Depositary Shares [Member]
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Shares issued | shares								34,500,000
Minimum [Member] | Sales Revenue, Net [Member] | Natural Gas, Oil and NGL [Member] | Customer Concentration Risk [Member]
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Concentration percentage														10.00%
Minimum [Member] | Accounting Standards Update 2016-02 [Member]
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Right-of-use asset				95,000,000					95,000,000			95,000,000				$ 95,000,000
Lease liability				95,000,000					95,000,000			95,000,000				95,000,000
Maximum [Member] | Accounting Standards Update 2016-02 [Member]
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Lease liability				$ 115,000,000					$ 115,000,000			115,000,000				$ 115,000,000
WPX Property Acquisition [Member]
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Percentage of interest acquired															86.00%
Fayetteville Shale [Member]
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Ownership interest prior to disposal		100.00%			100.00%
Consideration		$ 1,865,000,000			$ 1,865,000,000					1,865,000,000
Adjustment due to differences from economic effective date to close date	$ 215,000,000
Proceeds from sale of oil and gas property and equipment	$ 1,650,000,000
Discontinued Operations, Held-for-sale [Member]
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Impairments										160,000,000		160,000,000
Discontinued Operations, Held-for-sale [Member] | Midstream Services [Member]
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Impairments										145,000,000		145,000,000
Discontinued Operations, Held-for-sale [Member] | Exploration and Production [Member]
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Impairments										$ 15,000,000		15,000,000
Other Non-Core Assets [Member]
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Impairments												$ 11,000,000