Natural Gas Producing Activities (Unaudited) - Standard Measure of Discounted Future Cash Flow (Narrative) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($) $ / bbl	Dec. 31, 2019 USD ($) $ / MMcf	Dec. 31, 2019 USD ($) $ / MMBTU	Dec. 31, 2018 USD ($) $ / bbl $ / Dekatherm	Dec. 31, 2017 USD ($) $ / bbl $ / Dekatherm
Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Price used in computation of reserves	16.81
Future abandonment costs | $	$ 1,186	$ 1,186	$ 1,186	$ 883	$ 1,400
West Texas Intermediate
Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Price used in computation of reserves, gross	55.69
Price used in computation of reserves, adjustments	14.26
Price used in computation of reserves	41.43			65.56	51.34
NYMEX
Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Price used in computation of reserves	2.58	2.41	0.29
Columbia Gas Transmission Corp.
Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Price used in computation of reserves | $ / Dekatherm				2.888	2.801
Dominion Transmission, Inc.
Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Price used in computation of reserves | $ / Dekatherm				2.568	2.100
East Tennessee Natural Gas Pipeline
Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Price used in computation of reserves | $ / Dekatherm					2.914
Texas Eastern Transmission Corp
Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Price used in computation of reserves | $ / Dekatherm				2.587	2.058
Tennessee Zone 4-300 Leg of Tennessee Gas Pipeline Company
Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Price used in computation of reserves				2.320	1.995
Tennessee LA 500 Leg of Tennessee Gas Pipeline Company
Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Price used in computation of reserves | $ / Dekatherm					2.321
Waha
Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Price used in computation of reserves | $ / Dekatherm					2.665
Rockies Express Pipeline Zone 3
Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Price used in computation of reserves | $ / Dekatherm				2.939	2.840
West Virginia Marcellus reserves
Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Price used in computation of reserves				21.93	23.07
Kentucky
Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Price used in computation of reserves					31.11
Utica
Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Price used in computation of reserves				33.89	29.47
Permian
Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Price used in computation of reserves					27.93