4. Software and Other Intangible Assets

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.

At December 31, 2016, software net book value totaled $185.5 million, net of $195.0 million of accumulated amortization. Included in this amount is software marketed for external sale of $52.3 million. The remaining software net book value of $133.2 million is comprised of various software that has been acquired or developed for internal use.

Amortization of software marketed for external sale is computed using the greater of the ratio of current revenues to total current and anticipated revenues expected to be derived from the software or the straight-line method over an estimated useful life of generally three to ten years. Software for resale amortization expense recorded during the years ended December 31, 2017, 2016, and 2015, totaled $12.8 million, $13.9 million, and $14.5 million, respectively. These software amortization expense amounts are reflected in cost of revenue in the consolidated statements of operations.

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 amortization expense recorded during the years ended December 31, 2017, 2016, and 2015, totaled $45.2 million, $45.7 million, and $38.3 million, respectively. These software amortization expense amounts are reflected in depreciation and amortization in the 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):


	December 31, 2017							December 31, 2016
	Gross Carrying Amount		Accumulated Amortization			Net Balance		Gross Carrying Amount		Accumulated Amortization			Net Balance
Customer relationships	$	305,218	$	(116,677	)	$	188,541	$	295,730	$	(96,356	)	$	199,374
Trademarks and tradenames		16,646		(13,906	)		2,740		16,019		(11,759	)		4,260

	$321,864		$(130,583)			$191,281		$311,749		$(108,115)			$203,634

Other intangible assets amortization expense recorded during the years ended December 31, 2017, 2016, and 2015, totaled $19.4 million, $21.2 million, and $23.0 million, respectively.

Based on capitalized intangible assets at December 31, 2017, and assuming no impairment of these intangible assets, estimated amortization expense amounts in future fiscal years are as follows (in thousands):


Fiscal Year Ending December 31,	Software Amortization		Other Intangible Assets Amortization
2018	$	49,012	$	19,145
2019		40,471		18,587
2020		31,550		17,688
2021		19,734		17,176
2022		8,383		17,015
Thereafter		6,236		101,670

Total	$	155,386	$	191,281