Summary of significant accounting policies (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Future minimum lease payments
2015	$ 17
2016	15
2017	12
2018	9
2019	7
Thereafter	23
Total	83
Leases
Operating lease expense	$ 19	$ 19	$ 19
Composite annual depreciation rate (as a percent)	3.10%	3.10%	3.10%
Maximum
Cash and cash equivalents
Original maturity period of investments classified as cash equivalents	3 months
Equity method
Ownership percentage of affiliates classified as equity method investments	50.00%
Hawaiian Electric Company, Inc. and Subsidiaries
Future minimum lease payments
2015	$ 8
2016	6
2017	5
2018	4
2019	3
Thereafter	14
Total	40
Leases
Operating lease expense	$ 9	$ 8	$ 8