Commitments and contingencies - (Details) - HECO (USD $)	3 Months Ended		9 Months Ended			0 Months Ended	1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended								3 Months Ended		9 Months Ended				9 Months Ended		3 Months Ended				1 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012 item	Sep. 30, 2011	Nov. 01, 2012 Electric utility	Oct. 19, 2012 Hawaiian Electric Company, Inc. and Subsidiaries	Jan. 31, 2011 Hawaiian Electric Company, Inc. and Subsidiaries	Sep. 30, 2012 Hawaiian Electric Company, Inc. and Subsidiaries	Sep. 30, 2011 Hawaiian Electric Company, Inc. and Subsidiaries	Sep. 30, 2012 Hawaiian Electric Company, Inc. and Subsidiaries megawatt	Sep. 30, 2011 Hawaiian Electric Company, Inc. and Subsidiaries	Dec. 31, 2018 Hawaiian Electric Company, Inc. and Subsidiaries	Dec. 31, 2017 Hawaiian Electric Company, Inc. and Subsidiaries	Dec. 31, 2016 Hawaiian Electric Company, Inc. and Subsidiaries	Dec. 31, 2015 Hawaiian Electric Company, Inc. and Subsidiaries	Dec. 31, 2014 Hawaiian Electric Company, Inc. and Subsidiaries	Dec. 31, 2013 Hawaiian Electric Company, Inc. and Subsidiaries	Dec. 31, 2012 Hawaiian Electric Company, Inc. and Subsidiaries	Dec. 31, 2011 Hawaiian Electric Company, Inc. and Subsidiaries	Sep. 30, 2012 MECO	Sep. 30, 2011 MECO	Sep. 30, 2012 MECO	Sep. 30, 2011 MECO	Dec. 31, 2012 MECO	Mar. 31, 2012 MECO	Sep. 30, 2012 Campbell Industrial Park 110MW combustion turbine and transmission line Hawaiian Electric Company, Inc. and Subsidiaries	Sep. 30, 2012 East Oahu transmission project	Dec. 31, 2011 East Oahu transmission project Hawaiian Electric Company, Inc. and Subsidiaries	Sep. 30, 2012 East Oahu Transmission Project Phase 1	Sep. 30, 2012 East Oahu Transmission Project Phase 1 Hawaiian Electric Company, Inc. and Subsidiaries	Sep. 30, 2012 East Oahu Transmission Project Phase 2	Apr. 30, 2010 East Oahu Transmission Project Phase 2 Hawaiian Electric Company, Inc. and Subsidiaries	Sep. 30, 2012 East Oahu Transmission Project Phase 2 Hawaiian Electric Company, Inc. and Subsidiaries	Sep. 30, 2012 Customer Information System Project	Dec. 31, 2005 Customer Information System Project Hawaiian Electric Company, Inc. and Subsidiaries	Sep. 30, 2012 Customer Information System Project Hawaiian Electric Company, Inc. and Subsidiaries
Regulatory projects and legal obligations
Deferred cost to be recovered subject to PUC approval										$ 3,900,000
Changes in the asset retirement obligation liability
Balance at the beginning of the period							48,630,000			50,871,000	48,630,000
Accretion expense										1,233,000	1,641,000
Liabilities settled										(2,788,000)	(681,000)
Revisions in estimated cash flows											390,000
Balance at the end of the period								49,316,000	49,980,000	49,316,000	49,980,000
Integration from renewable energy sources (in megawatts)										200
Collective bargaining agreement
Percentage of employees who are members of the International Brotherhood of Electrical Workers					53.00%
Non-compounded wage increase (as a percent)												3.00%	3.00%	3.25%	3.00%	3.00%	3.00%
Additional deferred cost to be recovered subject to PUC approval										555,000
Maximum deferred cost recovery										5,890,000
Integration from firm renewable geothermal dispatchable energy (in megawatts))										50
Revenue recognized with respect to interim rate increase order										4,000,000
Project cost incurred																										195,000,000
Allowance for funds used during construction	(688,000)	(658,000)	(2,451,000)	(1,731,000)				(576,000)	(518,000)	(2,061,000)	(1,338,000)									(53,000)	(86,000)	(216,000)	(204,000)			9,000,000
Cost of project recoverable from increase in annual revenue																										163,000,000
Estimated project cost above PUC approval																										32,000,000
Estimated cost of project upon completion																											56,000,000		42,000,000		14,000,000
Accumulated project cost																														59,000,000
Estimated project cost subject to decision and order issued																														16,000,000
Write-off project cost																														9,500,000
Cost of settlement agreement																												6,000,000
Annual increase in cost of settlement agreement, subject to PUC approval																												5,000,000
Net estimated cost of using smart grid technology																																10,000,000
Estimated cost of smart grid technology																																15,000,000
Estimated statutory funding for smart grid technology																																5,000,000
Net actual cost of using smart grid technology																																	10,900,000
Cost of smart grid technology																																	15,400,000
Statutory funding for smart grid technology																																	4,500,000
Expected additional funding for smart grid technology																																	500,000
Amount authorized for the project																																			20,000,000
Amount authorized for project attributable to capital and deferred costs																																		18,000,000
Deferred and capital cost estimate																																				59,000,000
Limitation on deferred project operation and maintenance expenses																		2,300,000	2,300,000					600,000
Environmental regulation
Number of steam generating units on the island of Hawaii			5
Number of EGUs impacted by proposed rules of MATS			14
Percentage of reduction in GHG emissions by 2020						25.00%
Deferral period for GHG permitting requirements for carbon dioxide emissions							3 years
Additional accrued investigation and estimated cleanup costs																				$ 3,600,000		$ 3,600,000			$ 3,100,000
Period for which operations was stopped prior to merger			4 years