Discontinued Operations and Dispositions (Consolidated Statement of Operations) (Details) - USD ($) $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net income from discontinued operations before income taxes	[1]									$ 85	$ 32	$ 18
(Provision) benefit for income taxes	[1]									(33)	(2)	4
Net income from discontinued operations	[1]									52	30	22
Less: net (loss) income attributable to noncontrolling interests										(3)	(39)	5
Net income from discontinued operations attributable to MIC		$ (2)	$ 46	$ 5	$ 6	$ 1	$ 20	$ 11	$ 37	55	69	17
Bayonne Energy Center
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain (loss) on disposition of business										80	(17)
Contracted Power Segment | Discontinued Operations, Held-for-sale or Disposed of by Sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Product revenue										44	150	146
Cost of product sales										(7)	(24)	(21)
Selling, general & administrative expenses										(19)	(25)	(25)
Depreciation and amortization										0	(38)	(60)
Interest expense, net										(13)	(17)	(23)
Other (expense) income, net										80	(14)	1
Net income from discontinued operations before income taxes										85	32	18
(Provision) benefit for income taxes										(33)	(2)	4
Net income from discontinued operations										52	30	22
Less: net (loss) income attributable to noncontrolling interests										(3)	(39)	5
Net income from discontinued operations attributable to MIC										$ 55	$ 69	$ 17

[1] See Note 5, “Discontinued Operations and Dispositions”, for discussions on businesses classified as held for sale.