Variable Interest Entities - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016	Dec. 31, 2015
Variable Interest Entity [Line Items]
Noncontrolling interests on condensed consolidated balance sheets	$ 256		$ 257
Net income attributable to noncontrolling interests, Trust	1	$ 0
VIE, cash and cash equivalents	249	$ 16	882	$ 825
VIE. proved natural gas and oil properties	66,847		66,451
VIE. accumulated depreciation, depletion and amortization	62,934		62,726
VIE, other current liabilities	1,798
Variable Interest Entities, Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
VIE, cash and cash equivalents	1		1
VIE. proved natural gas and oil properties	488		488
VIE. accumulated depreciation, depletion and amortization	459		$ 458
VIE, other current liabilities	$ 3