Fair value measurements - (Details) - HECO (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Off-balance sheet items
Undiscounted future cash outflows used to compute fair value of asset retirement obligations	$ 33,000
Carrying or notional amount | Significant other observable inputs (Level 2)
Financial assets
Cash and cash equivalents	236,336	270,255
Financial liabilities
Long-term debt, net, including amounts due within one year	1,282,602	1,340,070
Carrying or notional amount | Hawaiian Electric Company, Inc. and Subsidiaries | Fair value measurements on a recurring basis | Significant other observable inputs (Level 2)
Financial assets
Cash and cash equivalents	5,850	48,806
Financial liabilities
Short-term borrowings - nonaffiliates	84,942
Long-term debt, net, including amounts due within one year	1,000,602	1,058,070
Off-balance sheet items
HECO-obligated preferred securities of trust subsidiary	50,000	50,000
Estimated fair value | Significant other observable inputs (Level 2)
Financial assets
Cash and cash equivalents	236,336	270,255
Financial liabilities
Long-term debt, net, including amounts due within one year	1,338,777	1,400,241
Estimated fair value | Hawaiian Electric Company, Inc. and Subsidiaries | Fair value measurements on a recurring basis | Significant other observable inputs (Level 2)
Financial assets
Cash and cash equivalents	5,850	48,806
Financial liabilities
Short-term borrowings - nonaffiliates	84,942
Long-term debt, net, including amounts due within one year	1,037,062	1,095,133
Off-balance sheet items
HECO-obligated preferred securities of trust subsidiary	$ 50,000	$ 50,000