Natural Gas and Oil Properties (Narrative) (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) derivative_position $ / bbl $ / MMBTU	Sep. 30, 2019 USD ($) derivative_position $ / bbl $ / MMBTU	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Jun. 30, 2019 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
Net book value adjusted for market differentials	$ 361		$ 361		$ 650	$ 1,480
Impairments	$ 361	$ 2	$ 2,495	$ 8
Number of hedge positions designated for hedge accounting | derivative_position			0	0
Other Non-Core Assets
Natural Gas and Oil Properties [Line Items]
Impairments			$ 5
Non-Core Leasehold
Natural Gas and Oil Properties [Line Items]
Consideration amount							$ 25
Natural Gas
Natural Gas and Oil Properties [Line Items]
Full cost ceiling test, price per MMBtu (in dollars per MMBtu) | $ / MMBTU			1.97	2.87
Oil
Natural Gas and Oil Properties [Line Items]
Full cost ceiling test, price per barrel (in dollars per bbl) | $ / bbl			43.40	57.77
NGL
Natural Gas and Oil Properties [Line Items]
Full cost ceiling test, price per barrel (in dollars per bbl) | $ / bbl			9.26	12.59