Basis of Presentation (Details) $ in Thousands			3 Months Ended		9 Months Ended
	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) shares	Sep. 30, 2016 USD ($) shares	Sep. 30, 2017 USD ($) item shares	Sep. 30, 2016 USD ($) 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 | $	$ 24,000	$ 21,700
Taxes Collected from Customers
Excise taxes and Hawaii Public Utility Commission fees | $			$ 2,100	$ 2,100	$ 6,300	$ 6,400
Denominator used to compute basic and diluted earnings per share
Basic earnings per share - weighted average shares			11,587,963	11,512,280	11,568,540	11,499,947
Effect of dilutive securities:
Employee and director restricted stock units (in shares)						39,881
Diluted earnings per share - weighted average shares			11,587,963	11,512,280	11,568,540	11,539,828
Shares attributable to restricted stock units excluded from computation of weighted average dilutive shares			311,090	227,888	311,090	8,319
Recently Adopted Accounting Pronouncements
Cumulative effect of adoption of new accounting standard for stock compensation | $					$ 550