4. Software and Other Intangible Assets

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.

At December 31, 2015, software net book value totaled $237.9 million, net of $158.9 million of accumulated amortization. Included in this amount is software marketed for external sale of $70.1 million. The remaining software net book value of $167.8 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, 2016, 2015, and 2014 totaled $13.9 million, $14.5 million, and $14.8 million, respectively. These software amortization expense amounts are reflected in cost of license in the consolidated statements of income.

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, 2016, 2015, and 2014 totaled $45.7 million, $38.3 million, and $26.7 million, respectively. These software amortization expense amounts are reflected in depreciation and amortization in the consolidated statements of income.

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, 2016							December 31, 2015
	Gross Carrying Amount		Accumulated Amortization			Net Balance		Gross Carrying Amount		Accumulated Amortization			Net Balance
Customer relationships	$	295,730	$	(96,356	)	$	199,374	$	336,075	$	(86,585	)	$	249,490
Trademarks and tradenames		16,019		(11,759	)		4,260		18,040		(10,605	)		7,435
Purchased Contracts		10,429		(10,429	)		—		10,690		(10,690	)		—

	$	322,178	$	(118,544	)	$	203,634	$	364,805	$	(107,880	)	$	256,925

Other intangible assets amortization expense recorded during the years ended December 31, 2016, 2015, and 2014 totaled $21.2 million, $23.0 million, and $24.7 million, respectively.

Based on capitalized intangible assets at December 31, 2016, 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
2017	$	55,229	$	19,040
2018		41,254		18,537
2019		32,871		18,006
2020		25,733		17,139
2021		17,848		16,677
Thereafter		12,561		114,235

Total	$	185,496	$	203,634