7. FIXED ASSETS:

As of December 31, 2018 and 2017, property, plant and equipment consisted of the following (in thousands):


	Useful Life
	(in Years)	2018		2017
Telecommunications equipment and towers	5 -15	$	914,276	$	774,548
Solar assets	20-23		57		113,218
Office and computer equipment	3 -10		77,085		76,706
Buildings	15-39		48,900		48,058
Transportation vehicles	3 -10		15,039		12,221
Leasehold improvements	Shorter of useful life or lease term		2,033		2,864
Land	—		14,728		12,516
Furniture and fixtures	5 -10		9,200		6,674
Total property, plant and equipment			1,081,318		1,046,805
Construction in progress			107,598		123,001
Total property, plant and equipment			1,188,916		1,169,806
Less: Accumulated depreciation			(562,064)		(526,660)
Net fixed assets		$	626,852	$	643,146

Depreciation and amortization of fixed assets, using the straight‑line method over the assets’ estimated useful life, for the years ended December 31, 2018, 2017 and 2016 was $83.0 million, $83.3 million and $73.3 million, respectively. Included within telecommunication equipment and towers are certain right to use assets under capital lease with a cost of $27.7 million and $30.0 million and net book value of and $20.2 million and $24.4 million, as of December 31, 2018 and 2017, respectively. Remaining amounts due under the Indefeasible Rights of Use (“IRUs”) are $1.3 million and $0.6 million as of December 31, 2018 and 2017, respectively.

For the years ended December 31, 2017 and 2016, amounts of capital expenditures were offset by grants of $1.5 million and $2.3 million, respectively. There was no such offset for the year ended December 31, 2018.

In 2018, the U.S. Telecom segment sold approximately 100 cell sites for $24.0 million. The disposed assets had a book value of $8.8 million and the Company recorded a gain of $15.2 million on the transaction.