Long-Term Debt (Narrative) (Details)	1 Months Ended	3 Months Ended
	Mar. 31, 2020 USD ($)	Mar. 31, 2020 USD ($)	Mar. 31, 2019 USD ($)	May 04, 2020 USD ($)	Mar. 17, 2020 USD ($)	Dec. 31, 2019 USD ($)
Debt Instrument [Line Items]
Letters of credit	$ 12,000,000	$ 12,000,000
Current and long-term debt	3,578,000,000	3,578,000,000				$ 2,707,000,000
Amortization of debt discount		1,000,000	$ 1,000,000
Revolving Credit Facility
Debt Instrument [Line Items]
Undrawn portion of line of credit	736,000,000	$ 736,000,000
Line of credit					$ 874,000,000
Proceeds from line of credit	$ 874,000,000
Convertible Senior Notes Due October 2023
Debt Instrument [Line Items]
Interest rate, stated percentage	2.00%	2.00%
Debt instrument, term		7 years
Current and long-term debt	$ 389,000,000	$ 389,000,000				388,000,000
Fair value of convertible senior notes	310,000,000	$ 310,000,000				370,000,000
Conversion rate		9.0290
Debt instrument, face amount	1,000	$ 1,000
Interest expense		3,000,000	3,000,000
Amortization of debt discount		1,000,000	$ 1,000,000
MIC Corporate
Debt Instrument [Line Items]
Current and long-term debt	988,000,000	988,000,000				388,000,000
MIC Corporate | Revolving Credit Facility
Debt Instrument [Line Items]
Proceeds from line of credit	599,000,000	599,000,000
Borrowing capacity	600,000,000	600,000,000
MIC Corporate | Convertible Senior Notes Due October 2023
Debt Instrument [Line Items]
Current and long-term debt	389,000,000	389,000,000				388,000,000
IMTT
Debt Instrument [Line Items]
Current and long-term debt	1,109,000,000	1,109,000,000				1,109,000,000
IMTT | Senior Notes
Debt Instrument [Line Items]
Borrowing capacity	600,000,000	600,000,000				600,000,000
Debt instrument, fair value disclosure	540,000,000	540,000,000				635,000,000
IMTT | Revolving Credit Facility
Debt Instrument [Line Items]
Borrowing capacity	600,000,000	600,000,000				600,000,000
IMTT | Tax Exempt Bonds
Debt Instrument [Line Items]
Borrowing capacity	509,000,000	509,000,000				509,000,000
Atlantic Aviation
Debt Instrument [Line Items]
Current and long-term debt	1,287,000,000	1,287,000,000				1,015,000,000
Atlantic Aviation | Revolving Credit Facility
Debt Instrument [Line Items]
Proceeds from line of credit	275,000,000	275,000,000
Borrowing capacity	350,000,000	$ 350,000,000
Debt instrument, term		5 years
Atlantic Aviation | Term Loan Facility
Debt Instrument [Line Items]
Debt instrument, term		7 years
Current and long-term debt	1,012,000,000	$ 1,012,000,000				1,015,000,000
Hawaii Gas Business | Senior Notes
Debt Instrument [Line Items]
Debt instrument, fair value disclosure	100,000,000	100,000,000				105,000,000
Long-term debt	100,000,000	100,000,000				100,000,000
Hawaii Gas Business | Revolving Credit Facility
Debt Instrument [Line Items]
Undrawn portion of line of credit	60,000,000	60,000,000				60,000,000
Hawaii Gas Business | Term Loan Facility
Debt Instrument [Line Items]
Borrowing capacity	80,000,000	80,000,000				80,000,000
Hawaii Gas Business | Term Loan Facility | Solar facilities
Debt Instrument [Line Items]
Current and long-term debt	$ 14,000,000	$ 14,000,000				$ 15,000,000
Subsequent Event | Atlantic Aviation
Debt Instrument [Line Items]
Letters of credit				$ 10,000,000