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Segment information: (Details) $ in Thousands	3 Months Ended		9 Months Ended
Segment information: (Details) $ in Thousands	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) segment	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)
Geographic information
Number of operating segments \| segment			1
Revenues	$ 142,302	$ 136,942	$ 424,205	$ 405,866
Long-lived assets, net	421,267		421,267		$ 368,946
On-net
Geographic information
Revenues	105,091	99,416	312,346	294,069
Off-net
Geographic information
Revenues	37,092	37,418	111,458	111,452
Non-core
Geographic information
Revenues	119	108	401	345
North America
Geographic information
Revenues	115,253	113,452	345,999	335,606
Long-lived assets, net	305,648		305,648		269,364
North America \| On-net
Geographic information
Revenues	82,730	80,176	247,679	236,636
North America \| Off-net
Geographic information
Revenues	32,416	33,172	97,951	98,670
North America \| Non-core
Geographic information
Revenues	107	104	369	300
Europe
Geographic information
Revenues	24,400	21,978	71,246	66,126
Europe \| On-net
Geographic information
Revenues	19,966	17,921	58,433	53,889
Europe \| Off-net
Geographic information
Revenues	4,422	4,053	12,781	12,192
Europe \| Non-core
Geographic information
Revenues	12	4	32	45
Europe and other
Geographic information
Long-lived assets, net	115,619		115,619		$ 99,582
South America
Geographic information
Revenues	613	144	1,378	252
South America \| On-net
Geographic information
Revenues	599	139	1,344	244
South America \| Off-net
Geographic information
Revenues	14	5	34	8
Asia Pacific
Geographic information
Revenues	2,026	1,368	5,572	3,882
Asia Pacific \| On-net
Geographic information
Revenues	1,786	1,180	4,880	3,300
Asia Pacific \| Off-net
Geographic information
Revenues	240	$ 188	692	$ 582
Africa
Geographic information
Revenues	10		10
Africa \| On-net
Geographic information
Revenues	$ 10		$ 10