Employee Benefit Plans (DB Plan Weighted Average Asset Allocation) (Details)	Dec. 31, 2019	Dec. 31, 2018
Other Plan Benefits
Defined Benefit Plan Disclosure [Line Items]
DB Plan weighted average asset allocations	100.00%	100.00%
IMTT | DB Plans Benefits
Defined Benefit Plan Disclosure [Line Items]
DB Plan weighted average asset allocations	100.00%	100.00%
Equity securities | Other Plan Benefits
Defined Benefit Plan Disclosure [Line Items]
DB Plan weighted average asset allocations	47.00%	45.00%
Equity securities | IMTT | DB Plans Benefits
Defined Benefit Plan Disclosure [Line Items]
DB Plan weighted average asset allocations	44.00%	42.00%
Fixed income securities | Other Plan Benefits
Defined Benefit Plan Disclosure [Line Items]
DB Plan weighted average asset allocations	43.00%	44.00%
Fixed income securities | IMTT | DB Plans Benefits
Defined Benefit Plan Disclosure [Line Items]
DB Plan weighted average asset allocations	40.00%	41.00%
Private equity | Other Plan Benefits
Defined Benefit Plan Disclosure [Line Items]
DB Plan weighted average asset allocations	2.00%	1.00%
Private equity | IMTT | DB Plans Benefits
Defined Benefit Plan Disclosure [Line Items]
DB Plan weighted average asset allocations	8.00%	7.00%
Global real estate fund | Other Plan Benefits
Defined Benefit Plan Disclosure [Line Items]
DB Plan weighted average asset allocations	5.00%	6.00%
Global real estate fund | IMTT | DB Plans Benefits
Defined Benefit Plan Disclosure [Line Items]
DB Plan weighted average asset allocations	4.00%	6.00%
Cash | Other Plan Benefits
Defined Benefit Plan Disclosure [Line Items]
DB Plan weighted average asset allocations	3.00%	4.00%
Cash | IMTT | DB Plans Benefits
Defined Benefit Plan Disclosure [Line Items]
DB Plan weighted average asset allocations	4.00%	4.00%
Hawaii Gas Business | MIC Hawaii | DB Plans Benefits
Defined Benefit Plan Disclosure [Line Items]
DB Plan weighted average asset allocations	100.00%	100.00%
Hawaii Gas Business | Equity securities | MIC Hawaii | DB Plans Benefits
Defined Benefit Plan Disclosure [Line Items]
DB Plan weighted average asset allocations	26.00%	23.00%
Hawaii Gas Business | Fixed income securities | MIC Hawaii | DB Plans Benefits
Defined Benefit Plan Disclosure [Line Items]
DB Plan weighted average asset allocations	69.00%	71.00%
Hawaii Gas Business | Private equity | MIC Hawaii | DB Plans Benefits
Defined Benefit Plan Disclosure [Line Items]
DB Plan weighted average asset allocations	0.00%	0.00%
Hawaii Gas Business | Global real estate fund | MIC Hawaii | DB Plans Benefits
Defined Benefit Plan Disclosure [Line Items]
DB Plan weighted average asset allocations	4.00%	4.00%
Hawaii Gas Business | Cash | MIC Hawaii | DB Plans Benefits
Defined Benefit Plan Disclosure [Line Items]
DB Plan weighted average asset allocations	1.00%	2.00%