7. FIXED ASSETS:

As of December 31, 2017 and 2016, property, plant and equipment consisted of the following (in thousands):


	Useful Life
	(in Years)	2017		2016
Telecommunications equipment and towers	5 -15	$	774,548	$	802,415
Solar assets	20-23		113,218		115,932
Office and computer equipment	3 -10		76,706		44,147
Buildings	15-39		48,058		43,609
Transportation vehicles	3 -10		12,221		12,043
Leasehold improvements	Shorter of useful life or lease term		2,864		325


Land	—		12,516		10,242
Furniture and fixtures	5 -10		6,674		7,657
Total property, plant and equipment			1,046,805		1,036,370
Construction in progress			123,001		101,992
Total property, plant and equipment			1,169,806		1,138,362
Less: Accumulated depreciation			(526,660)		(490,650)
Net fixed assets		$	643,146		647,712

Depreciation and amortization of fixed assets, using the straight‑line method over the assets’ estimated useful life, for the years ended December 31, 2017, 2016 and 2015 was $83.3 million, $73.3 million and $55.9 million, respectively. Included within telecommunication equipment and towers are certain right to use assets under capital lease with a cost of $30.0 million and $13.8 million and net book value of and $24.4 million and $12.4 million, as of December 31, 2017 and 2016, respectively. Remaining amounts due under the IRUs are $0.6 million and $1.2 million as of December 31, 2017 and 2016, respectively.

During the year ended December 31, 2015, we sold certain network assets and telecommunications licenses in our U.S. Telecom segment and recognized a gain on such disposition of $2.8 million.

For the years ended December 31, 2017, 2016 and 2015, amounts of capital expenditures were offset by grants of $1.5 million, $2.3 million and $2.6 million, respectively.