Employee Benefit Plans (Amounts Not Yet Reflected in Net Periodic Benefit Cost and Included in Accumulated Other Comprehensive Loss) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Amounts not yet reflected in net periodic benefit cost and included in accumulated other comprehensive loss:
Accumulated loss	$ (33)	$ (21)
Accumulated other comprehensive loss	(33)	(21)
Net periodic benefit cost in excess (deficit) of cumulative employer contributions	(64)	(56)
Net amount recognized in balance sheet	(97)	(77)
Other Plan Benefits
Amounts not yet reflected in net periodic benefit cost and included in accumulated other comprehensive loss:
Accumulated loss	(5)	(4)
Accumulated other comprehensive loss	(5)	(4)
Net periodic benefit cost in excess (deficit) of cumulative employer contributions	(16)	(15)
Net amount recognized in balance sheet	(21)	(19)
IMTT | DB Plans Benefits
Amounts not yet reflected in net periodic benefit cost and included in accumulated other comprehensive loss:
Accumulated loss	(20)	(7)
Accumulated other comprehensive loss	(20)	(7)
Net periodic benefit cost in excess (deficit) of cumulative employer contributions	(54)	(48)
Net amount recognized in balance sheet	(74)	(55)
Hawaii Gas Business | MIC Hawaii | DB Plans Benefits
Amounts not yet reflected in net periodic benefit cost and included in accumulated other comprehensive loss:
Accumulated loss	(8)	(10)
Accumulated other comprehensive loss	(8)	(10)
Net periodic benefit cost in excess (deficit) of cumulative employer contributions	6	7
Net amount recognized in balance sheet	$ (2)	$ (3)