Description of the business and summary of significant accounting policies: (Details) $ in Millions	12 Months Ended
	Dec. 31, 2015 USD ($) MB	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Revenue recognition
Bad debt expense, net of recoveries | $	$ 3.3	$ 3.7	$ 3.5
Excise and Sales Taxes | $	$ 3.6	$ 0.2	$ 0.0
Minimum
On-net service - high-speed Internet access and IP connectivity
Speed per second of bandwidth (in megabits) | MB	100
Maximum
On-net service - high-speed Internet access and IP connectivity
Speed per second of bandwidth (in megabits) | MB	100