Unconsolidated variable interest entities (Details) - HECO (USD $) In Millions, unless otherwise specified	1 Months Ended	6 Months Ended
	Mar. 31, 2004 item	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011
Hawaiian Electric Company, Inc. and Subsidiaries
Power purchase agreement
Number of power purchase agreements (PPAs)		6
Maximum capacity of small power production facilities (in kilowatts)		100
Power purchase capacity excluding agreements with smaller IPPs (in megawatts)		548
HECO Capital Trust III
Unconsolidated variable interest entities
Investment in 2004 Debentures		$ 51.5		$ 51.5
Interest income		1.7	1.7
HECO Capital Trust III | Hawaiian Electric Company, Inc. and Subsidiaries
Unconsolidated variable interest entities
Principal amount of 2004 Debentures	31.5
HECO Capital Trust III | Hawaiian Electric Company, Inc. and Subsidiaries | 2004 Trust Preferred Securities
Unconsolidated variable interest entities
Number of 2004 Trust Preferred Securities issued	2,000,000
Dividend rate on 2004 Trust Preferred Securities (as a percent)	6.50%
Aggregate Liquidation preference	50.0
Balance of Trust Securities		50.0		50.0
Dividend distributions on Trust Preferred Securities		1.6	1.6
HECO Capital Trust III | Hawaiian Electric Company, Inc. and Subsidiaries | Trust Common Securities
Unconsolidated variable interest entities
Aggregate Liquidation preference	1.5
Balance of Trust Securities		1.5		1.5
Common dividend		0.1	0.1
HECO Capital Trust III | Hawaii Electric Light Company, Inc. (HELCO)
Unconsolidated variable interest entities
Principal amount of 2004 Debentures	10.0
HECO Capital Trust III | Maui Electric Company, Limited (MECO)
Unconsolidated variable interest entities
Principal amount of 2004 Debentures	$ 10.0