Property and Equipment

12 Months Ended

Dec. 31, 2015

Property, Plant and Equipment [Abstract]

Property and equipment consisted of the following as of December 31, 2015 and 2014 (dollars in thousands):



	December 31, 2015			December 31, 2014			Estimated Useful Life in Years
Computer and networking equipment	$	1,046,739		$	850,533		3-7
Purchased software	46,509			46,537			3-10
Furniture and fixtures	35,212			27,923			5
Office equipment	21,108			14,035			3-5
Leasehold improvements	119,466			92,544			1-16
Internal-use software	546,520			448,777			2-7
Property and equipment, gross	1,815,554			1,480,349
Accumulated depreciation and amortization	(1,062,374		)	(878,758		)
Property and equipment, net	$	753,180		$	601,591

Depreciation and amortization expense on property and equipment and capitalized internal-use software for the years ended December 31, 2015, 2014 and 2013 was $272.5 million, $215.3 million and $162.9 million, respectively. During the years ended December 31, 2015, 2014 and 2013, the Company capitalized $17.9 million, $15.2 million and $12.3 million, respectively, of stock-based compensation related to employees who developed and enhanced internal-use software applications.

During the years ended December 31, 2015 and 2014, the Company wrote off $48.7 million and $100.1 million, respectively, of property and equipment, gross, along with the associated accumulated depreciation and amortization. The write-offs were primarily related to computer and networking equipment and internal-use software no longer in use. These assets had been substantially depreciated and amortized.