Property and equipment: Exchange agreement (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property and equipment
Non-cash component of network equipment obtained in exchange transactions	$ 3,861	$ 7,739	$ 8,156
Network equipment
Property and equipment
Non-cash component of network equipment obtained in exchange transactions	3,900	7,700	5,300
Network equipment \| Estimate \| Level 3
Property and equipment
Fair value of equipment	$ 9,100	$ 15,500	$ 17,900

Fair value portion of physical assets used in the normal conduct of business to produce goods and services and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, and furniture and fixtures.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19190-110258

Name:	us-gaap_PropertyPlantAndEquipmentFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant