Organization and Summary of Significant Accounting Policies (Narrative) (Details)		1 Months Ended			3 Months Ended	12 Months Ended
	Nov. 13, 2020 USD ($) $ / shares shares	Aug. 31, 2020 USD ($) $ / shares shares	Jan. 31, 2018 USD ($) shares	Jan. 31, 2015 shares	Sep. 30, 2018 USD ($)	Dec. 31, 2020 USD ($) derivative_position subsidiary segment $ / bbl $ / MMBTU shares	Dec. 31, 2019 USD ($) derivative_position subsidiary $ / shares $ / bbl $ / MMBTU shares	Dec. 31, 2018 USD ($) derivative_position $ / shares $ / bbl $ / MMBTU shares	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2015
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Number of segments | segment						2
Outstanding checks included in accounts payable						$ 16,000,000	$ 15,000,000
Natural gas, oil and NGL reserves discount						10.00%
Net unevaluated costs excluded from amortization, cumulative						$ 1,472,000,000	$ 1,506,000,000
Net book value adjusted for market differentials						$ 335,000,000			$ 361,000,000	$ 650,000,000	$ 1,479,000,000
Period of time needed to calculate ceiling value of reserves						12 months	12 months	12 months
Number of derivative positions designated for hedge accounting | derivative_position						0	0	0
Impairments						$ 2,830,000,000	$ 16,000,000	$ 171,000,000
Preferred stock dividend			$ 27,000,000			$ 0	0	27,000,000
Treasury stock acquired							$ 21,000,000	$ 180,000,000
Treasury stock (in shares) | shares							5,260,687	39,061,268
Treasury stock acquired, average cost per share (in dollars per share) | $ / shares							$ 3.84	$ 4.63
Shares held in trust (in shares) | shares						3,632	5,115
Discontinued operations, held-for-sale
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Impairments					$ 161,000,000			$ 160,000,000
Common Stock
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Shares issued (in shares) | shares						63,250,000
Marketing-Related Intangible Assets
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Intangible assets, current						$ 48,000,000	$ 56,000,000
Amortization of intangible asset						9,000,000	$ 9,000,000	$ 9,000,000
Expected amortization in year one						8,000,000
Expected amortization in year two						5,000,000
Expected amortization in year three						5,000,000
Expected amortization in year four						5,000,000
Expected amortization in year five						$ 5,000,000
Natural Gas
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Average sales price ($ per unit) | $ / MMBTU						1.98	2.58	3.10
Oil
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Average sales price ($ per unit) | $ / bbl						39.57	55.69	65.56
NGL
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Average sales price ($ per unit) | $ / bbl						10.27	11.58	17.64
Depositary Shares
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Shares issued (in shares) | shares				34,500,000
Common Stock
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Shares issued upon conversion (in shares) | shares			74,998,614
Montage Resources Corporation
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Number of Southwestern Energy common stock for each share of Montage Resources Corporation common stock converted (in shares) | shares	1.8656	1.8656
Percentage of interest acquired	90.00%
Net unevaluated costs excluded from amortization, cumulative						$ 88,000,000
Impairments						0
Unamortized cost of properties acquired						1,087,000,000
Offering price (in dollars per share) | $ / shares	$ 3.05	$ 2.50
Shares of Southwestern common stock issued in respect of outstanding Montage common stock and stock-based awards | shares	69,740,848
Total consideration (fair value of Southwestern common shares issued)	$ 213,000,000
Montage Resources Corporation | Pro Forma
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Impairments						$ 539,000,000
Montage Resources Corporation | Common Stock
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Offering price (in dollars per share) | $ / shares	$ 3.05
Shares of Southwestern common stock issued in respect of outstanding Montage common stock and stock-based awards | shares	69,740,848
Total consideration (fair value of Southwestern common shares issued)	$ 213,000,000
Montage Resources Corporation | Public Stock Offering
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Underwritten public offering of common stock (in shares) | shares		63,250,000
Net proceeds from public offering		$ 152,000,000
WPX Property Acquisition
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Percentage of interest acquired												86.00%
Non-full cost pool assets | Minimum
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Long lived assets, useful life						3 years
Non-full cost pool assets | Maximum
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Long lived assets, useful life						30 years
Other non-core assets
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Impairments					$ 1,000,000	$ 5,000,000	$ 16,000,000	$ 11,000,000
Revenue Benchmark | Customer concentration risk
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Number of subsidiaries of major customer with which business is conducted | subsidiary						1	0
Concentration percentage						10.00%