Description of the business and summary of significant accounting policies: (Tables)

12 Months Ended

Dec. 31, 2017

Description of the business and summary of significant accounting policies:

Schedule of depreciation and amortization periods

Type of asset	Depreciation or amortization period
Indefeasible rights of use (IRUs)	Shorter of useful life or the IRU lease agreement; generally 15 to 20 years
Network equipment	3 to 8 years
Leasehold improvements	Shorter of lease term, including reasonably assured renewal periods, or useful life
Software	5 years
Owned buildings	40 years
Office and other equipment	3 to 7 years
System infrastructure	5 to 10 years

Schedule of diluted weighted average shares

	Year Ended December 31, 2017		Year Ended December 31, 2016		Year Ended December 31, 2015
Weighted average common shares—basic		44,855,263		44,641,805		44,888,723
Dilutive effect of stock options		31,534		31,760		40,196
Dilutive effect of restricted stock		297,406		199,465		230,570
Weighted average common shares—diluted		45,184,203		44,873,030		45,159,489

Schedule of unvested and anti-dilutive shares

	December 31, 2017		December 31, 2016		December 31, 2015
Unvested shares of restricted common stock		1,112,151		908,761		870,751
Anti-dilutive options for common stock		47,513		90,959		119,872
Anti-dilutive shares of restricted common stock		201		79,450		362,241