Note 9—Other Assets, Net

Other assets, net of accumulated amortization, consisted of the following as of the dates indicated (in millions):

	December 31,
	2014			2013
Intangible assets	$	676		$	674
Debt issue costs	92			70
Fair value of derivative instruments	27			30
Other	19			37
	814			811
Accumulated amortization	(325		)	(271		)
	$	489		$	540

Costs incurred in connection with the issuance of long-term debt and amendments to our credit facilities are capitalized and amortized using the straight-line method over the term of the related debt. Use of the straight-line method does not differ materially from the “effective interest” method of amortization. Fully amortized debt issue costs and the related accumulated amortization are written off in conjunction with the refinancing or termination of the applicable debt arrangement. We capitalized debt issue costs of $24 million and $9 million in 2014 and 2013, respectively. Gross debt issue costs of $2 million and $8 million were removed from our Consolidated Balance Sheet during 2014 and 2013, respectively.

Amortization expense related to other assets (including finite-lived intangible assets) for the three years ended December 31, 2014, 2013 and 2012 was $64 million, $96 million and $99 million, respectively. Amortization expense for finite-lived intangible assets for the years ended December 31, 2014, 2013 and 2012 was $57 million, $85 million and $90 million, respectively.

Intangible assets that have finite lives are tested for impairment when events or circumstances indicate that the carrying value may not be recoverable. Our intangible assets that have finite lives consisted of the following as of the dates indicated (in millions):

		December 31, 2014							December 31, 2013
	Estimated Useful			Accumulated							Accumulated
	Lives (Years)	Cost		Amortization			Net		Cost		Amortization			Net
Customer contracts and relationships	1 - 20	$	593	$	(288	)	$	305	$	591	$	(237	)	$	354
Property tax abatement	7 - 13	38		(18		)	20		38		(14		)	24
Other agreements	25 - 70	37		(4		)	33		37		(3		)	34
Emission reduction credits (1)	N/A	8		—			8		8		—			8
		$	676	$	(310	)	$	366	$	674	$	(254	)	$	420

(1)

Emission reduction credits, once surrendered in exchange for environmental permits, are finite-lived.

We estimate that our amortization expense related to finite-lived intangible assets for the next five years will be as follows (in millions):

2015	$	52
2016	$	44
2017	$	41
2018	$	36
2019	$	33