Basis of Presentation (Details) $ in Millions			3 Months Ended
	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)	Mar. 31, 2018 USD ($) item shares	Mar. 31, 2017 USD ($) shares
Cash and Cash Equivalents
Number of banks in which the entity has its majority cash balances held in demand deposit accounts | item			1
Supplemental Non-Cash Investing and Financing Activities
Accounts payable | $	$ 24.8	$ 21.9
Taxes Collected from Customers
Excise taxes and Hawaii Public Utility Commission fees | $			$ 2.1	$ 2.2
Denominator used to compute basic and diluted earnings per share
Basic earnings per share - weighted average shares			11,597,918	11,529,046
Diluted earnings per share - weighted average shares			11,597,918	11,529,046
Shares attributable to restricted stock units excluded from computation of weighted average dilutive shares			388,101	278,701
Maximum
Supplemental Non-Cash Investing and Financing Activities
Period of IRU related payments to be received			25 years