Summary of Significant Accounting Policies (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) item	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Summary of Significant Accounting Policies
Advance billing period for basic local service and enhanced calling feature services	1 month
Taxes Collected from Customers
Excise taxes and Hawaii Public Utility Commission fees	$ 8.4	$ 8.4	$ 8.2
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	$ 27.4	$ 26.1	$ 20.6