Organization and Summary of Significant Accounting Policies (Narrative) (Details)	1 Months Ended					3 Months Ended	9 Months Ended	12 Months Ended			132 Months Ended	168 Months Ended
	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Jul. 31, 2016 USD ($) $ / shares shares	Nov. 30, 2015	Jan. 31, 2015 USD ($) $ / shares shares	Mar. 31, 2017 USD ($)	Sep. 30, 2017 shares	Dec. 31, 2017 USD ($) segment item $ / shares $ / bbl $ / MMBTU shares	Dec. 31, 2016 USD ($) item $ / shares $ / bbl $ / MMBTU shares	Dec. 31, 2015 USD ($) item $ / bbl $ / MMBTU	Dec. 31, 2014 USD ($)	Dec. 31, 2017 USD ($) $ / shares shares	Jan. 12, 2018 shares	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Number of segments | segment								2
Unamortized debt expense									$ 28,000,000
Outstanding checks included in accounts payable								$ 17,000,000	$ 8,000,000			$ 17,000,000
Natural gas, oil and NGL reserves discount								10.00%	10.00%	10.00%
Period of time needed to calculate ceiling value of reserves								12 months	12 months	12 months
Net unevaluated costs excluded from amortization								$ 214,000,000	$ 66,000,000	$ 210,000,000	$ 1,327,000,000	$ 1,817,000,000
Impairment of natural gas and oil properties, net of tax		$ 506,000,000												$ 297,000,000	$ 641,000,000	$ 1,746,000,000	$ 944,000,000
Number of derivative position designated for hedge accounting | item								0	0	0
Cash flow hedges impact on ceiling value, net of tax																$ 40,000,000	$ 60,000,000
Amount recognized is the largest amount of tax benefit, threshold								50.00%
Repayments of long-term debt								$ 1,139,000,000	$ 1,175,000,000	$ 500,000,000
Common stock, date dividend declared								Dec. 18, 2017
Common stock, shares issued as stock dividend | shares							10,040,306		9,917,799
Treasury stock, shares | shares								31,269	31,269			31,269
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								10.30%
Accounting Standards Update 2016-09 [Member]
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Net cumulative-effect adjustment						$ 59,000,000
Long-term Debt [Member]
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Unamortized debt expense								$ 40,000,000	$ 28,000,000	24,000,000		$ 40,000,000
Long-term Debt [Member] | 2015 Term Loan due December 2020 [Member]
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Unamortized debt expense									2,000,000
Repayments of long-term debt	$ 327,000,000	$ 48,000,000	$ 375,000,000					$ 327,000,000	423,000,000
Debt instrument face amount									$ 750,000,000
Debt instrument, term				3 years
Senior Notes [Member]
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Repayments of long-term debt			$ 700,000,000
Proceeds from issuance of debt					$ 2,200,000,000
Bridge Loan [Member]
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Bridge loan										$ 4,500,000,000
Debt instrument, term										364 days
Natural Gas [Member]
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Full cost ceiling test, price | $ / MMBTU								2.98	2.48	2.59
Oil [Member]
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Full cost ceiling test, price | $ / bbl								47.79	39.25	46.79
NGL [Member]
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Impairment of natural gas and oil properties, net of tax										$ 1,586,000,000
Full cost ceiling test, price | $ / bbl								14.41	6.74	6.82
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
United States [Member]
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Net unevaluated costs excluded from amortization								$ 1,817,000,000
Gathering Systems [Member]
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Straight-line depreciation period								25 years
Common Stock [Member]
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Shares issued | shares			98,900,000		30,000,000
Price per share | $ / shares			$ 13.00		$ 23.00
Proceeds from issuance of shares			$ 1,247,000,000		$ 669,000,000
Depositary Shares [Member]
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Shares issued | shares					34,500,000
Proceeds from issuance of shares					$ 1,700,000,000
Depositary shares conversion rate					0.05%
Liquidation preference per share | $ / shares					$ 50
Series B Preferred Stock [Member]
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Liquidation preference per share | $ / shares					$ 1,000			$ 1,000	$ 1,000			$ 1,000
WPX Property Acquisition [Member]
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Percentage of interest acquired										86.00%
Subsequent Event [Member]
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Shares issued upon conversion | shares													74,998,614