5. Software and Other Intangible Assets

At June 30, 2018, software net book value totaling $144.7 million, net of $239.5 million of accumulated amortization, includes the net book value of software marketed for external sale of $33.8 million. The remaining software net book value of $110.9 million is comprised of various software that has been acquired or developed for internal use.

At December 31, 2017, software net book value totaled $155.4 million, net of $230.7 million of accumulated amortization. Included in this amount is software marketed for external sale of $40.9 million. The remaining software net book value of $114.5 million is comprised of various software that has been acquired or developed for internal use.

Quarterly amortization of software marketed for external sale is computed using the greater of the ratio of current revenues to total estimated revenues expected to be derived from the software or the straight-line method over an estimated useful life of three to ten years. Software for resale amortization expense recorded in the three months ended June 30, 2018 and 2017, totaled $3.4 million and $3.2 million, respectively. Software for resale amortization expense recorded in the six months ended June 30, 2018 and 2017, totaled $7.0 million and $6.5 million, respectively. These software amortization expense amounts are reflected in cost of revenue in the condensed consolidated statements of operations.

Quarterly amortization of software for internal use is computed using the straight-line method over an estimated useful life of three to ten years. Software for internal use includes software acquired through acquisitions that is used to provide certain of our SaaS and PaaS offerings. Amortization of software for internal use in the three months ended June 30, 2018 and 2017, totaled $10.3 million and $11.3 million, respectively. Amortization of software for internal use in the six months ended June 30, 2018 and 2017, totaled $20.8 million and $22.6 million, respectively. These software amortization expense amounts are included in depreciation and amortization in the condensed consolidated statements of operations.

The carrying amount and accumulated amortization of the Company’s other intangible assets that were subject to amortization at each balance sheet date are as follows:


	June 30, 2018							December 31, 2017
(in thousands)	Gross Carrying Amount		Accumulated Amortization			Net Balance		Gross Carrying Amount		Accumulated Amortization			Net Balance
Customer relationships	$	302,080	$	(124,465	)	$	177,615	$	305,218	$	(116,677	)	$	188,541
Trademarks and tradenames		16,507		(14,550	)		1,957		16,646		(13,906	)		2,740

	$318,587		$(139,015)			$179,572		$321,864		$(130,583)			$191,281

Other intangible assets amortization expense for the three months ended June 30, 2018 and 2017, totaled $4.8 million. Other intangible assets amortization expense for the six months ended June 30, 2018 and 2017, totaled $9.7 million and $9.6 million, respectively.

Based on capitalized software and other intangible assets at June 30, 2018, estimated amortization expense for future fiscal years is as follows:


Fiscal Year Ending December 31,	Software Amortization		Other Intangible Assets Amortization
(in thousands)
Remainder of 2018	$	25,561	$	9,401
2019		44,993		18,400
2020		35,162		17,509
2021		22,213		17,007
2022		9,157		16,853
2023		4,778		16,535
Thereafter		2,801		83,867

Total	$	144,665	$	179,572