Termination of proposed merger and other matters (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended			6 Months Ended	12 Months Ended
	Jul. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Jun. 30, 2016	Dec. 31, 2016
Hawaiian Electric Company, Inc. and Subsidiaries
Business Acquisition [Line Items]
Merger termination, expenses recognized			$ 1	$ 1
NextEra Energy, Inc Merger
Business Acquisition [Line Items]
Merger contract termination fee	$ 90
Maximum expenses paid to party for cancellation of merger	$ 5
Gain (loss) recognized, merger termination						$ 60
Merger termination, merger-and-spin-off related expenses, net of taxes		$ 64	$ (2)	$ (2)		$ (6)
Merger termination, termination fee		55
Merger termination, insurance reimbursements		$ 3
Recognition of previously disallowed merger and acquisition expenses					$ 8