Income Taxes (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2016	Jun. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax [Line Items]
Tax benefit related to unrecognized tax benefits					$ 42
Valuation allowance against U.S. deferred tax assets, percent				100.00%		100.00%	100.00%
Gain on sale of aggregate ownership interests, before-tax					2,607		$ (13)
Deferred tax benefit resulting from release of valuation allowance position; allocated to discontinued operations					$ 259
U.S. statutory income tax rate					21.00%	35.00%	35.00%
Deferred income tax expense (benefit)					$ 226	$ (97)	$ 43
Capital loss carryforward, deferred tax asset		$ 760			609	760
Tax expense related to unrecognized tax benefits							63
Deferred tax assets, valuation allowance		968			640	968
Net operating loss carryforwards, deferred tax assets		796			287	796
Unrecognized tax benefits, interest and penalties		28			12	28
Unrecognized tax benefit that would impact effective tax rate					70
Unrecognized tax benefits removed					43	7
Interest associated with tax examinations					$ 20
United States Federal [Member]
Income Tax [Line Items]
Net operating loss carryforward, expiration date					Dec. 31, 2037
Net operating loss carryforwards					$ 389
Various U.S. States [Member]
Income Tax [Line Items]
Net operating loss carryforwards					$ 784
Minimum [Member] | United States Federal [Member]
Income Tax [Line Items]
Operating loss carryforward, utilization period					Dec. 31, 2019
Minimum [Member] | Various U.S. States [Member]
Income Tax [Line Items]
Net operating loss carryforward, expiration date					Dec. 31, 2019
Operating loss carryforward, utilization period					Dec. 31, 2019
Minimum [Member] | Canada Federal [Member]
Income Tax [Line Items]
Net operating loss carryforward, expiration date					Dec. 31, 2029
Operating loss carryforward, utilization period					Dec. 31, 2019
Maximum [Member] | Various U.S. States [Member]
Income Tax [Line Items]
Net operating loss carryforward, expiration date					Dec. 31, 2038
Maximum [Member] | Canada Federal [Member]
Income Tax [Line Items]
Net operating loss carryforward, expiration date					Dec. 31, 2038
Canada [Member]
Income Tax [Line Items]
Change in deferred tax valuation allowance		641			$ (59)
Capital loss carryforward, deferred tax asset		727				727
Deferred tax assets, valuation allowance					609		71
Net operating loss carryforwards					595
U.S. [Member]
Income Tax [Line Items]
Change in deferred tax valuation allowance						$ (323)	$ 313
Deferred tax assets, valuation allowance					31
Allocated goodwill			$ 83
U.S. [Member] | Transition Tax [Member]
Income Tax [Line Items]
Deferred income tax expense (benefit)		167
U.S. [Member] | Change in Income Tax Rate [Member]
Income Tax [Line Items]
Deferred income tax expense (benefit)		$ 108
EnLink and General Partner [Member]
Income Tax [Line Items]
Gain on sale of aggregate ownership interests, before-tax	$ 2,600				$ 2,600