5. Software and Other Intangible Assets

At September 30, 2018, software net book value totaling $147.3 million, net of $250.0 million of accumulated amortization, includes the net book value of software marketed for external sale of $30.9 million. The remaining software net book value of $116.4 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 September 30, 2018 and 2017, totaled $2.6 million and $3.1 million, respectively. Software for resale amortization expense recorded in the nine months ended September 30, 2018 and 2017, totaled $9.6 million. 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 September 30, 2018 and 2017, totaled $10.2 million and $11.5 million, respectively. Amortization of software for internal use in the nine months ended September 30, 2018 and 2017, totaled $31.0 million and $34.1 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:


(in thousands)	September 30, 2018							December 31, 2017
	Gross Carrying Amount		Accumulated Amortization			Net Balance		Gross Carrying Amount		Accumulated Amortization			Net Balance
Customer relationships	$	300,782	$	(128,367	)	$	172,415	$	305,218	$	(116,677	)	$	188,541
Trademarks and tradenames		16,450		(14,808	)		1,642		16,646		(13,906	)		2,740

	$	317,232	$	(143,175	)	$	174,057	$	321,864	$	(130,583	)	$	191,281

Other intangible assets amortization expense for the three months ended September 30, 2018 and 2017, totaled $4.7 million and $4.9 million, respectively. Other intangible assets amortization expense for the nine months ended September 30, 2018 and 2017, totaled $14.4 million and $14.5 million, respectively.

Based on capitalized software and other intangible assets at September 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	$	13,724	$	4,670
2019		47,416		18,325
2020		37,964		17,438
2021		24,930		16,939
2022		11,957		16,789
2023		6,306		16,478
Thereafter		5,019		83,418

Total	$	147,316	$	174,057