Natural Gas and Oil Properties (Narrative) (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2021 USD ($) derivative_position	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) derivative_position $ / bbl $ / MMBTU	Sep. 30, 2020 USD ($) $ / bbl $ / MMBTU	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)
Natural Gas and Oil Properties [Line Items]
Natural gas, oil and NGL reserves discount			10.00%
Period of time needed to calculate ceiling value of reserves			12 months
Number of hedge positions designated for hedge accounting | derivative_position	0		0
Impairments	$ 6,000,000	$ 361,000,000	$ 6,000,000	$ 2,495,000,000
Net book value adjusted for market differentials		$ 361,000,000		$ 361,000,000	$ 650,000,000	$ 1,500,000,000
Montage Resources Corporation
Natural Gas and Oil Properties [Line Items]
Impairments			0
Unamortized cost of properties acquired	1,183,000,000		1,183,000,000
Montage Resources Corporation | Pro Forma
Natural Gas and Oil Properties [Line Items]
Impairments	$ 0		$ 0
Natural Gas
Natural Gas and Oil Properties [Line Items]
Full cost ceiling test, price per MMBtu (in dollars per MMBtu) | $ / MMBTU			2.94	1.97
Oil
Natural Gas and Oil Properties [Line Items]
Full cost ceiling test, price per barrel (in dollars per bbl) | $ / bbl			57.69	43.40
NGL
Natural Gas and Oil Properties [Line Items]
Full cost ceiling test, price per barrel (in dollars per bbl) | $ / bbl			23.26	9.26