Natural Gas Producing Activities (Unaudited) - Standard Measure of Discounted Future Cash Flow (Narrative) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($) $ / bbl $ / Dekatherm	Dec. 31, 2015 $ / bbl $ / Dekatherm	Dec. 31, 2014 $ / bbl $ / Dekatherm
Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Discount rate to compute standard measure of future cash flow (as a percent)	10.00%
Future Net Cash Flows Relating to Proved Oil and Gas Reserves, Net Cash Flows [Abstract]
Discounted future net cash flows relating to proved oil and gas reserves, change in price of natural gas sensitivity (in usd per dth)	0.2
Discounted future net cash flows relating to proved oil and gas reserves, change in price of oil sensitivity (in usd per bbl) | $ / bbl	10
Discounted future net cash flows relating to proved oil and gas reserves, change in price of natural gas liquids (in usd per bbl) | $ / bbl	10
Change in discounted future cash flows for assumed natural gas price change | $	$ 1,100.0
Change in discounted future cash flows for assumed oil price change | $	28.0
Change in discounted future cash flows for assumed natural gas liquids price change | $	$ 715.1
West Texas Intermediate
Future Net Cash Flows Relating to Proved Oil and Gas Reserves, Net Cash Flows [Abstract]
Price used in computation of reserves (in usd per bbl) | $ / bbl	42.75	50.28	94.99
Columbia Gas Transmission Corp.
Future Net Cash Flows Relating to Proved Oil and Gas Reserves, Net Cash Flows [Abstract]
Price used in computation of oil and gas reserves (in usd per dth)	2.342	2.506	4.278
Dominion Transmission, Inc.
Future Net Cash Flows Relating to Proved Oil and Gas Reserves, Net Cash Flows [Abstract]
Price used in computation of oil and gas reserves (in usd per dth)	1.348	1.394	3.191
East Tennessee Natural Gas Pipeline
Future Net Cash Flows Relating to Proved Oil and Gas Reserves, Net Cash Flows [Abstract]
Price used in computation of oil and gas reserves (in usd per dth)	2.334	2.552	4.350
Texas Eastern Transmission Corp
Future Net Cash Flows Relating to Proved Oil and Gas Reserves, Net Cash Flows [Abstract]
Price used in computation of oil and gas reserves (in usd per dth)	1.325	1.428	3.258
Tennessee Zone 4-300 Leg of Tennessee Gas Pipeline Company
Future Net Cash Flows Relating to Proved Oil and Gas Reserves, Net Cash Flows [Abstract]
Price used in computation of oil and gas reserves (in usd per dth)	1.305	1.079	2.286
Tennessee LA 500 Leg of Tennessee Gas Pipeline Company
Future Net Cash Flows Relating to Proved Oil and Gas Reserves, Net Cash Flows [Abstract]
Price used in computation of oil and gas reserves (in usd per dth)	1.862	2.430	4.170
Waha
Future Net Cash Flows Relating to Proved Oil and Gas Reserves, Net Cash Flows [Abstract]
Price used in computation of oil and gas reserves (in usd per dth)	2.343	2.473	4.152
Rockies Express Pipeline Zone 3
Future Net Cash Flows Relating to Proved Oil and Gas Reserves, Net Cash Flows [Abstract]
Price used in computation of oil and gas reserves (in usd per dth) | $ / bbl	2.402
Houston Ship Channel
Future Net Cash Flows Relating to Proved Oil and Gas Reserves, Net Cash Flows [Abstract]
Price used in computation of oil and gas reserves (in usd per dth)		2.549	4.243
West Virginia Marcellus reserves
Future Net Cash Flows Relating to Proved Oil and Gas Reserves, Net Cash Flows [Abstract]
Price used in computation of reserves (in usd per bbl) | $ / bbl	13.87	17.60	49.22
Kentucky
Future Net Cash Flows Relating to Proved Oil and Gas Reserves, Net Cash Flows [Abstract]
Price used in computation of reserves (in usd per bbl) | $ / bbl	17.27	21.69	49.47
Utica
Future Net Cash Flows Relating to Proved Oil and Gas Reserves, Net Cash Flows [Abstract]
Price used in computation of reserves (in usd per bbl) | $ / bbl	14.71	16.84	47.11
Permian
Future Net Cash Flows Relating to Proved Oil and Gas Reserves, Net Cash Flows [Abstract]
Price used in computation of reserves (in usd per bbl) | $ / bbl	18.91	17.51	31.92