Employee Benefit Plans (Components of Net Periodic Benefit Cost and Other Changes in Other Comprehensive Loss (Income) for Plans) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Components of net periodic benefit cost:
Service cost	$ 7	$ 8	$ 7
Interest cost	9	9	9
Expected return on plan assets	(8)	(10)	(9)
Recognized actuarial loss	1	1	1
Special termination benefits	0	0	1
Net periodic benefit cost	9	8	9
Other changes recognized in other comprehensive loss (income):
Net (gain) loss arising during the year	13	(11)	8
Amortization of loss	(1)	(1)	(1)
Total recognized in other comprehensive (income) loss	12	(12)	7
Other Plan Benefits
Components of net periodic benefit cost:
Service cost	1	1	1
Interest cost	1	1	1
Expected return on plan assets	0	(1)	0
Recognized actuarial loss	0	0	0
Special termination benefits	0	0	0
Net periodic benefit cost	2	1	2
Other changes recognized in other comprehensive loss (income):
Net (gain) loss arising during the year	1	(1)	1
Amortization of loss	0	0	0
Total recognized in other comprehensive (income) loss	$ 1	$ (1)	$ 1
Weighted average assumptions to determine benefit obligations:
Rate of compensation increase	4.57%	4.57%	4.57%
Weighted average assumptions to determine net cost:
Expected long-term rate of return on plan assets during fiscal year	5.50%	5.75%	5.75%
Rate of compensation increase	4.57%	4.57%	4.57%
Other Plan Benefits | Maximum
Weighted average assumptions to determine benefit obligations:
Discount rate	3.35%	4.35%	3.70%
Weighted average assumptions to determine net cost:
Discount rate	4.35%	3.70%	4.25%
Assumed healthcare cost trend rates:
Initial health care cost trend rate	7.75%	8.50%	7.00%
Ultimate rate	5.00%	5.00%	5.00%
Other Plan Benefits | Minimum
Weighted average assumptions to determine benefit obligations:
Discount rate	2.82%	3.91%	3.25%
Weighted average assumptions to determine net cost:
Discount rate	3.91%	3.25%	3.56%
Assumed healthcare cost trend rates:
Initial health care cost trend rate	7.25%	8.00%	6.98%
Ultimate rate	4.50%	4.50%	4.50%
IMTT | DB Plans Benefits
Components of net periodic benefit cost:
Service cost	$ 5	$ 6	$ 5
Interest cost	6	6	6
Expected return on plan assets	(5)	(6)	(6)
Recognized actuarial loss	0	0	0
Special termination benefits	0	0	1
Net periodic benefit cost	6	6	6
Other changes recognized in other comprehensive loss (income):
Net (gain) loss arising during the year	13	(11)	9
Amortization of loss	0	0	0
Total recognized in other comprehensive (income) loss	$ 13	$ (11)	$ 9
Weighted average assumptions to determine benefit obligations:
Discount rate	3.25%	4.35%	3.70%
Rate of compensation increase	4.57%	4.57%	4.57%
Weighted average assumptions to determine net cost:
Discount rate	4.35%	3.70%	4.30%
Expected long-term rate of return on plan assets during fiscal year	5.50%	5.60%	6.25%
Rate of compensation increase	4.57%	4.57%	4.57%
Hawaii Gas Business | MIC Hawaii | DB Plans Benefits
Components of net periodic benefit cost:
Service cost	$ 1	$ 1	$ 1
Interest cost	2	2	2
Expected return on plan assets	(3)	(3)	(3)
Recognized actuarial loss	1	1	1
Special termination benefits	0	0	0
Net periodic benefit cost	1	1	1
Other changes recognized in other comprehensive loss (income):
Net (gain) loss arising during the year	(1)	1	(2)
Amortization of loss	(1)	(1)	(1)
Total recognized in other comprehensive (income) loss	$ (2)	$ 0	$ (3)
Weighted average assumptions to determine benefit obligations:
Discount rate	3.20%	4.25%	3.60%
Weighted average assumptions to determine net cost:
Discount rate	4.25%	3.60%	4.00%
Expected long-term rate of return on plan assets during fiscal year	4.90%	5.90%	5.90%