Description of the business and recent developments: (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2013 MB
Financial instruments
Letters of credit, outstanding amount	0.4
Minimum
On-net service - high-speed Internet access and IP connectivity
Speed per second of bandwidth (in megabits)	100
Maximum
On-net service - high-speed Internet access and IP connectivity
Speed per second of bandwidth (in megabits)	10,240
Convertible senior notes
Financial instruments
Aggregate principal amount of debt issued	92.0
Convertible senior notes \| Level 1
Financial instruments
Debt instrument, amount	89.9
Senior notes
Financial instruments
Aggregate principal amount of debt issued	175.0
Senior notes \| Level 2
Financial instruments
Debt instrument, amount	193.9

The stated principal amount of the debt instrument at time of issuance, which may vary from the carrying amount because of unamortized premium or discount.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 835

-SubTopic 30

-Section 45

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28551-108399

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 835

-SubTopic 30

-Section 55

-Paragraph 8

-URI http://asc.fasb.org/extlink&oid=6584090&loc=d3e28878-108400

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher AICPA

-Name Accounting Principles Board Opinion (APB)

-Number 21

-Paragraph 16, 20

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Name:	us-gaap_DebtInstrumentFaceAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant