Description of the business and summary of significant accounting policies: (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2013 Minimum MB	Dec. 31, 2013 Maximum MB	Dec. 31, 2013 Convertible senior notes	Jun. 30, 2007 Convertible senior notes	Dec. 31, 2013 Convertible senior notes Level 2	Dec. 31, 2013 Senior notes	Aug. 19, 2013 Senior notes	Jan. 26, 2011 Senior notes	Dec. 31, 2013 Senior notes Level 2
Description of the business and summary of significant accounting policies:
Bad debt expense, net of recoveries	$ 3.5	$ 5.1	$ 4.3
Financial instruments
Aggregate principal amount of debt issued						92.0	200.0		240.0	65.0	175.0
Debt instrument, amount								89.2				260.7
Letters of credit, outstanding amount	$ 0.4	$ 0.4
On-net service - high-speed Internet access and IP connectivity
Speed per second of bandwidth (in megabits/gigabits)				100	10,240