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Segment information: (Details) $ in Thousands	3 Months Ended		9 Months Ended
Segment information: (Details) $ in Thousands	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) segment	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)
Geographic information
Number of operating segments \| segment			1
Revenues	$ 147,927	$ 142,302	$ 442,584	$ 424,205
Long-lived assets, net	454,722		454,722		$ 430,351
On-net
Geographic information
Revenues	111,099	105,091	332,087	312,346
Off-net
Geographic information
Revenues	36,656	37,092	110,079	111,458
Non-core
Geographic information
Revenues	172	119	418	401
North America
Geographic information
Revenues	117,661	115,253	351,198	345,999
Long-lived assets, net	326,350		326,350		306,652
North America \| On-net
Geographic information
Revenues	85,569	82,730	255,157	247,679
North America \| Off-net
Geographic information
Revenues	31,945	32,416	95,680	97,951
North America \| Non-core
Geographic information
Revenues	147	107	361	369
Europe
Geographic information
Revenues	26,062	24,400	79,972	71,246
Europe \| On-net
Geographic information
Revenues	21,658	19,966	66,450	58,433
Europe \| Off-net
Geographic information
Revenues	4,379	4,422	13,465	12,781
Europe \| Non-core
Geographic information
Revenues	25	12	57	32
Asia Pacific
Geographic information
Revenues	2,934	2,026	8,091	5,572
Asia Pacific \| On-net
Geographic information
Revenues	2,657	1,786	7,279	4,880
Asia Pacific \| Off-net
Geographic information
Revenues	277	240	812	692
Europe and other
Geographic information
Long-lived assets, net	128,372		128,372		$ 123,699
Africa
Geographic information
Revenues	174	10	345	10
Africa \| On-net
Geographic information
Revenues	173	10	342	10
Africa \| Off-net
Geographic information
Revenues	1		3
South America
Geographic information
Revenues	1,096	613	2,978	1,378
South America \| On-net
Geographic information
Revenues	1,042	599	2,859	1,344
South America \| Off-net
Geographic information
Revenues	$ 54	$ 14	$ 119	$ 34