Employee Benefit Plans (Schedule of Additional Information) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Change in benefit obligation:
Benefit obligation – beginning of year	$ 220	$ 244
Service cost	7	8	$ 7
Interest cost	9	9	9
Actuarial losses (gains)	30	(28)
Benefits paid	(14)	(13)
Benefit obligation – end of year	252	220	244
Change in plan assets:
Fair value of plan assets - beginning of year	143	162
Actual return on plan assets	25	(7)
Employer contributions	1	1
Benefits paid	(14)	(13)
Fair value of plan assets – end of year	155	143	162
Other Plan Benefits
Change in benefit obligation:
Benefit obligation – beginning of year	28	29
Service cost	1	1	1
Interest cost	1	1	1
Actuarial losses (gains)	2	(2)
Benefits paid	(2)	(1)
Benefit obligation – end of year	30	28	29
Change in plan assets:
Fair value of plan assets - beginning of year	9	9
Actual return on plan assets	1	0
Employer contributions	1	1
Benefits paid	(2)	(1)
Fair value of plan assets – end of year	9	9	9
IMTT | DB Plans Benefits
Change in benefit obligation:
Benefit obligation – beginning of year	143	162
Service cost	5	6	5
Interest cost	6	6	6
Actuarial losses (gains)	23	(22)
Benefits paid	(9)	(9)
Benefit obligation – end of year	168	143	162
Change in plan assets:
Fair value of plan assets - beginning of year	88	102
Actual return on plan assets	15	(5)
Employer contributions	0	0
Benefits paid	(9)	(9)
Fair value of plan assets – end of year	94	88	102
Hawaii Gas Business | MIC Hawaii | DB Plans Benefits
Change in benefit obligation:
Benefit obligation – beginning of year	49	53
Service cost	1	1	1
Interest cost	2	2	2
Actuarial losses (gains)	5	(4)
Benefits paid	(3)	(3)
Benefit obligation – end of year	54	49	53
Change in plan assets:
Fair value of plan assets - beginning of year	46	51
Actual return on plan assets	9	(2)
Employer contributions	0	0
Benefits paid	(3)	(3)
Fair value of plan assets – end of year	$ 52	$ 46	$ 51