6. Intangible Assets

Our intangible assets are comprised of both indefinite-lived and finite-lived intangible assets. Intangible assets with indefinite lives and goodwill are not amortized, but are reviewed for impairment at least annually or whenever events or circumstances indicate the carrying value of the asset may not be recoverable. We perform our annual impairment tests of our indefinite-lived intangible assets and goodwill during the fourth quarter of each fiscal year. We also reevaluate the useful life of the indefinite-lived intangible assets each reporting period to determine whether events and circumstances continue to support an indefinite useful life. The results of our annual indefinite-lived intangible assets and goodwill impairment assessments in the fourth quarter of 2016 indicated no impairment.

As of September 30, 2017 and December 31, 2016, our goodwill balance, all of which related to our BA segment, was $0.6 million.

Our intangible assets, other than goodwill, as of September 30, 2017 and December 31, 2016 were as follows (in thousands, except for weighted average remaining useful life):


	Weighted
	Average		As of September 30, 2017							As of December 31, 2016
	Remaining		Gross					Net		Gross					Net
	Useful Life		Carrying		Accumulated			Carrying		Carrying		Accumulated			Carrying
	(in years)		Amount		Amortization			Amount		Amount		Amortization			Amount
Amortized intangible assets:
Software		2.2	$	140,818	$	(85,490	)	$	55,328	$	118,836	$	(70,127	)	$	48,709
Service customer relationship		2.5		8,081		(5,534	)		2,547		8,081		(4,773	)		3,308
Other intangible assets		1.3		1,500		(997	)		503		1,500		(682	)		818
OEM and dealer relationships				6,724		(6,724	)		—		6,724		(6,667	)		57

Total amortized intangible assets				157,123		(98,745	)		58,378		135,141		(82,249	)		52,892
Unamortized intangible assets:
FCC Licenses				32,283		—			32,283		32,283		—			32,283

Total intangible assets			$	189,406	$	(98,745	)	$	90,661	$	167,424	$	(82,249	)	$	85,175

Amortization expense was $4.9 million and $16.5 million, respectively, for the three and nine month periods ended September 30, 2017, and $6.0 million and $15.8 million, respectively, for the prior year periods.

Amortization expense for each of the next five years and thereafter is estimated to be as follows (in thousands):


	Amortization
Years ending December 31,	Expense
2017 (period from October 1 to December 31)	$	7,137
2018	$	24,284
2019	$	14,427
2020	$	8,382
2021	$	2,425
Thereafter	$	1,723

Actual future amortization expense could differ from the estimated amount as a result of future investments and other factors.