6. FIXED ASSETS:

As of December 31, 2016 and 2015, property, plant and equipment consisted of the following (in thousands):


	Useful Life
	(in Years)	2016		2015
Telecommunications equipment and towers	5 -15	$	802,415	$	553,237
Solar assets	20-23		115,932		111,446
Office and computer equipment	3 -10		44,147		54,665
Buildings	15-39		43,609		18,540
Transportation vehicles	3 -10		12,043		8,882
Leasehold improvements	Shorter of useful life or lease term		325		11,592


Land	—		10,242		1,198
Furniture and fixtures	5 -10		7,657		6,584
Total property, plant and equipment			1,036,370		766,144
Construction in progress			101,992		41,103
Total property, plant and equipment			1,138,362		807,247
Less: Accumulated depreciation			(490,650)		(433,744)
Net fixed assets		$	647,712		373,503

Depreciation and amortization of fixed assets, using the straight‑line method over the assets’ estimated useful life, for the years ended December 31, 2016, 2015 and 2014 was $73.3 million, $55.9 million and $50.3 million, respectively. Included within telecommunication equipment and towers are assets related to Indefeasible Rights of Use under capital lease with a cost of $13.8 million and $2.7 million and net book value of and $12.4 million and $1.8 million, as of December 31, 2016 and 2015, respectively. Remaining amounts due under the IRUs are $1.2 million and $0.0 million as of December 31, 2016 and 2015, 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, 2016, 2015 and 2014, amounts of capital expenditures were offset by grants of $2.3 million, $2.6 million and $2.3 million, respectively.