Goodwill and Intangible Assets

Goodwill

The changes in the carrying value of goodwill for the years ended December 31, 2018 and December 31, 2017 were as follows:



Balance as of December 31, 2016	$	5,758
Additions due to FiberNet Acquisition^(a)	778
Additions due to Wilcon Acquisition^(a)	358
Additions due to Lightower Acquisition^(a)	3,115
Adjustments due to other acquisitions, purchase price allocations and other, net	12
Balance as of December 31, 2017	$	10,021
Adjustments due to other acquisitions, purchase price allocations and other, net	57
Balance as of December 31, 2018	$	10,078


(a)	The final purchase price allocations for the FiberNet Acquisition, Wilcon Acquisition and Lightower Acquisition resulted in the recognition of goodwill in the Fiber segment based on:


•	the Company's expectation to leverage the FiberNet, Wilcon and Lightower fiber footprint to support new small cells and fiber solutions,


•	the complementary nature of the FiberNet, Wilcon and Lightower fiber to the Company's existing fiber assets and its location where the Company expects to see wireless carrier network investments,


•	the Company's belief that the acquired fiber assets are well-positioned to benefit from the continued growth trends in the demand for data, and


•	other intangibles not qualified for separate recognition, including the assembled workforce. See note 3.

Intangibles

The following is a summary of the Company's intangible assets. See note 3 for further discussion of the Company's acquisitions.



	As of December 31, 2018							As of December 31, 2017
	Gross Carrying Value^(a)		Accumulated Amortization			Net Book Value		Gross Carrying Value		Accumulated Amortization			Net Book Value
Site rental contracts and tenant relationships	$	7,787	$	(2,578	)	$	5,209	$	7,782	$	(2,156	)	$	5,626
Other intangible assets	494		(187		)	307		504		(168		)	336
Total	$	8,281	$	(2,765	)	$	5,516	$	8,286	$	(2,324	)	$	5,962


(a)	During the year ended December 31, 2018, intangible assets additions (primarily site rental contracts and tenant relationships) from acquisitions had a weighted average amortization period of approximately 20 years.

Amortization expense related to intangible assets is classified as follows on the Company's consolidated statement of operations and comprehensive income (loss):



	For Years Ended December 31,
Classification	2018		2017		2016
Depreciation, amortization and accretion	$	428	$	314	$	265
Site rental costs of operations	17		18		19
Total amortization expense	$	445	$	332	$	284

The estimated annual amortization expense related to intangible assets (inclusive of those recorded as an increase to "site rental costs of operations") for the years ending December 31, 2019 to 2023 is as follows:



	Years Ending December 31,
	2019		2020		2021		2022		2023
Estimated annual amortization	$	445	$	444	$	444	$	443	$	443