Variable Interest Entities - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016	Dec. 31, 2015
Variable Interest Entity [Line Items]
Noncontrolling interests on condensed consolidated balance sheets	$ 254		$ 254		$ 257
Net income attributable to noncontrolling interests, Trust	1	$ 0	2	$ 0
VIE, cash and cash equivalents	13	$ 4	13	$ 4	882	$ 825
VIE. proved natural gas and oil properties	67,419		67,419		66,451
VIE. accumulated depreciation, depletion and amortization	63,138		63,138		62,726
VIE, other current liabilities	1,297		1,297
Variable Interest Entities, Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
VIE, cash and cash equivalents	1		1		1
VIE. proved natural gas and oil properties	488		488		488
VIE. accumulated depreciation, depletion and amortization	459		459		$ 458
VIE, other current liabilities	$ 4		$ 4