PROPERTY AND EQUIPMENT

12 Months Ended

Dec. 31, 2017

Property, Plant and Equipment [Abstract]

Property and equipment consisted of the following (in thousands):



	December 31,
	2017			2016
Network equipment	$	247,190		$	231,579
Network equipment under capital lease	14,206			14,231
Furniture and equipment	26,246			18,300
Software	43,930			48,011
Leasehold improvements	412,631			396,891
Buildings under capital lease	227,482			63,117
Property and equipment, gross	971,685			772,129
Less: accumulated depreciation and amortization ($50,253 and $40,218 related to capital leases at December 31, 2016 and 2015, respectively)	(513,120		)	(469,449		)
	$	458,565		$	302,680

We disposed or retired $9.2 million of property and equipment with accumulated depreciation of $7.3 million during the year ended December 31, 2017, $5.0 million of assets with accumulated depreciation of $4.4 million during the year ended December 31, 2016 and $33.3 million of assets with accumulated depreciation of $32.6 million during the year ended December 31, 2015.

We capitalized an immaterial amount of interest for each of the three years ended December 31, 2017. Also, during the year ended December 31, 2017, we determined that we would not use certain leasehold improvements from our recently exited data center property and recorded an impairment of $0.5 million. At the time of disposal, the leasehold improvements had a cost of $22.4 million with accumulated depreciation of $22.4 million. During the year ended December 31, 2016, we determined that we would not use certain internally-developed software related to our quoting and billing system and recorded an impairment of $1.6 million. At the time of disposal, the software had a cost of $2.4 million with accumulated depreciation of $0.8 million.

Depreciation and amortization of property and equipment consisted of the following (in thousands):



	Year ended December 31,
	2017		2016		2015
Costs of sales and services	$	70,368	$	71,626	$	70,080
Other depreciation and amortization	2,478		2,274		19,125
Subtotal	72,846		73,900		89,205
Amortization of acquired and developed technologies	2,147		3,048		3,450
Total depreciation and amortization	$	74,993	$	76,948	$	92,655