Long-Term Debt (Narrative) (Details)			1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
		Oct. 13, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)	Jan. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Debt Instrument [Line Items]
Letters of credit				$ 13,000,000		$ 13,000,000
Interest expense	[1]			36,000,000	$ 28,000,000	124,000,000	$ 71,000,000
Amortization of debt discount						3,000,000	3,000,000
Current and long-term debt			$ 3,064,000,000	2,708,000,000		2,708,000,000		$ 3,064,000,000
Revolving Credit Facility
Debt Instrument [Line Items]
Undrawn portion of line of credit				$ 1,610,000,000		$ 1,610,000,000
Convertible Senior Notes Due July 2019
Debt Instrument [Line Items]
Interest rate, stated percentage			2.875%					2.875%
Convertible senior notes			$ 350,000,000					$ 350,000,000
Debt instrument, term								5 years
Convertible Senior Notes Due October 2023
Debt Instrument [Line Items]
Interest rate, stated percentage				2.00%		2.00%
Convertible senior notes			384,000,000	$ 387,000,000		$ 387,000,000		$ 384,000,000
Debt instrument, term								7 years
Fair value of convertible senior notes				355,000,000		355,000,000
Interest expense				3,000,000	10,000,000	3,000,000	10,000,000
Amortization of debt discount				1,000,000	$ 4,000,000	1,000,000	$ 4,000,000
Subsequent Event | Convertible Senior Notes Due October 2023
Debt Instrument [Line Items]
Conversion rate		9.0290
Debt instrument, face amount		$ 1,000
MIC Corporate
Debt Instrument [Line Items]
Current and long-term debt			734,000,000	387,000,000		387,000,000		$ 734,000,000
MIC Corporate | Revolving Credit Facility
Debt Instrument [Line Items]
Undrawn portion of line of credit			600,000,000	600,000,000		600,000,000		600,000,000
Borrowing capacity									$ 600,000,000	$ 410,000,000
MIC Corporate | Convertible Senior Notes Due October 2023
Debt Instrument [Line Items]
Convertible senior notes			403,000,000	403,000,000		403,000,000		403,000,000
Current and long-term debt			384,000,000	387,000,000		387,000,000		384,000,000
IMTT
Debt Instrument [Line Items]
Current and long-term debt			1,109,000,000	1,109,000,000		1,109,000,000		1,109,000,000
IMTT | Senior Notes
Debt Instrument [Line Items]
Current and long-term debt				600,000,000		600,000,000
Debt instrument, fair value disclosure				630,000,000		630,000,000
IMTT | Revolving Credit Facility
Debt Instrument [Line Items]
Undrawn portion of line of credit			600,000,000	600,000,000		600,000,000		600,000,000
Borrowing capacity			600,000,000					600,000,000
IMTT | Tax Exempt Bonds
Debt Instrument [Line Items]
Borrowing capacity			$ 509,000,000					$ 509,000,000
Percentage of variable rate			80.00%					80.00%
IMTT | London Interbank Offered Rate (LIBOR) | Tax Exempt Bonds
Debt Instrument [Line Items]
Fixed portion of interest rate component			0.45%
Atlantic Aviation
Debt Instrument [Line Items]
Current and long-term debt			$ 1,025,000,000	1,017,000,000		$ 1,017,000,000		$ 1,025,000,000
Atlantic Aviation | Revolving Credit Facility
Debt Instrument [Line Items]
Debt instrument, term						5 years		5 years
Borrowing capacity			$ 350,000,000	350,000,000		$ 350,000,000		$ 350,000,000
Atlantic Aviation | Term Loan Facility
Debt Instrument [Line Items]
Debt instrument, term			7 years
Borrowing capacity			$ 1,025,000,000					$ 1,025,000,000
Hawaii Gas Business | Senior Notes
Debt Instrument [Line Items]
Current and long-term debt				100,000,000		100,000,000
Debt instrument, fair value disclosure				105,000,000		105,000,000
Hawaii Gas Business | Revolving Credit Facility
Debt Instrument [Line Items]
Undrawn portion of line of credit				60,000,000		60,000,000
Hawaii Gas Business | Term Loan Facility
Debt Instrument [Line Items]
Current and long-term debt				80,000,000		80,000,000
Hawaii Gas Business | Term Loan Facility | Solar facilities
Debt Instrument [Line Items]
Current and long-term debt				$ 15,000,000		$ 15,000,000

[1] Interest expense includes losses on derivative instruments of $2 million and $14 million for the quarter and nine months ended September 30, 2019, respectively. Interest expense includes gains on derivative instruments of $3 million and $17 million for the quarter and nine months ended September 30, 2018, respectively.