Quarterly financial information (unaudited): (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly financial information (unaudited):
Service revenue	$ 125,226	$ 122,969	$ 119,777	$ 117,203	$ 115,596	$ 113,057	$ 109,955	$ 108,291	$ 485,175	$ 446,900	$ 404,234
Network operations, including equity-based compensation expense	53,918	53,584	51,115	50,662	50,089	48,827	47,872	47,277	209,278	194,066	174,510
Gains on equipment transactions	319	397	1,023	2,124	667	687	4,439	1,946	3,862	7,739	5,443
Operating income	20,534	17,720	19,000	18,666	14,795	16,063	17,511	15,675	75,918	64,042	53,036
Net income (loss)	$ (6,227)	$ 3,650	$ 4,317	$ 4,136	$ 3,892	$ 3,459	$ 4,224	$ 3,354	$ 5,876	$ 14,929	$ 4,896
Net income (loss) per common share-basic and diluted	$ (0.14)	$ 0.08	$ 0.10	$ 0.09	$ 0.09	$ 0.08	$ 0.09	$ 0.08
Weighted-average number of common shares-basic	44,844,469	44,767,163	44,717,372	44,649,645	44,577,826	44,574,583	44,491,899	44,402,640	44,855,263	44,641,805	44,888,723
Weighted-average number of common shares-diluted	44,844,469	45,118,607	44,988,655	44,917,014	44,803,782	44,816,860	44,757,494	44,571,937	45,184,203	44,873,030	45,159,489

Total costs related to services rendered by an entity during the reporting period.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.2(d))
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The amount of net income or loss for the period per each share in instances when basic and diluted earnings per share are the same amount and reported as a single line item on the face of the financial statements. Basic earnings per share is the amount of net income or loss for the period per each share of common stock or unit outstanding during the reporting period. Diluted earnings per share includes the amount of net income or loss for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

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Amount of gain (loss) on sale or disposal of property, plant and equipment assets, including oil and gas property and timber property.

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Net income after adjustments for dividends on preferred stock (declared in the period) and/or cumulative preferred stock (accumulated for the period).

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Total revenue from sale of goods and services rendered during the reporting period, in the normal course of business, reduced by sales returns and allowances, and sales discounts.

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The average number of shares or units issued and outstanding that are used in calculating diluted EPS or earnings per unit (EPU), determined based on the timing of issuance of shares or units in the period.

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