5. Software and Other Intangible Assets

At March 31, 2018, software net book value totaling $150.7 million, net of $236.8 million of accumulated amortization, includes the net book value of software marketed for external sale of $37.9 million. The remaining software net book value of $112.8 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 March 31, 2018 and 2017 totaled $3.6 million and $3.3 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 of $10.5 million and $11.3 million for the three months ended March 31, 2018 and 2017, respectively, is 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)	March 31, 2018							December 31, 2017
	Gross Carrying Amount		Accumulated Amortization			Net Balance		Gross Carrying Amount		Accumulated Amortization			Net Balance
Customer relationships	$	308,489	$	(122,164	)	$	186,325	$	305,218	$	(116,677	)	$	188,541
Trademarks and tradenames		16,842		(14,479	)		2,363		16,646		(13,906	)		2,740

	$	325,331	$	(136,643	)	$	188,688	$	321,864	$	(130,583	)	$	191,281

Other intangible assets amortization expense for the three months ended March 31, 2018 and 2017 totaled $4.9 million and $4.8 million, respectively.

Based on capitalized software and other intangible assets at March 31, 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	$	38,406	$	14,410
2019		42,955		18,777
2020		33,127		17,866
2021		20,461		17,342
2022		8,780		17,184
2023		4,640		16,854
Thereafter		2,284		86,255

Total	$	150,653	$	188,688