Note 8—Other Assets, Net

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

	December 31,
	2013			2012
Debt issue costs	$	70		$	69
Fair value of derivative instruments	30			10
Intangible assets	674			642
Other	37			54
	811			775
Accumulated amortization	(271		)	(189		)
	$	540		$	586

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 approximately $9 million and $20 million in 2013 and 2012, respectively. Approximately $8 million and $5 million of gross debt issue costs, primarily related to the restructuring of credit facilities, were removed from our Consolidated Balance Sheet during 2013 and 2012, respectively.

Amortization expense related to other assets (including finite-lived intangible assets) for the three years ended December 31, 2013, 2012 and 2011 was approximately $96 million, $99 million and $44 million, respectively. Our amortization expense for finite-lived intangible assets for the years ended December 31, 2013, 2012 and 2011 was approximately $85 million, $90 million and $36 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 consist of the following as of the dates indicated (in millions):

		December 31, 2013							December 31, 2012
	Estimated Useful			Accumulated							Accumulated
	Lives (Years)	Cost		Amortization			Net		Cost		Amortization			Net
Customer contracts and relationships	1 - 20	$	591	$	(237	)	$	354	$	558	$	(157	)	$	401
Property tax abatement	7 - 13	38		(14		)	24		38		(10		)	28
Other agreements	25 - 70	37		(3		)	34		38		(2		)	36
Emission reduction credits (1)	N/A	8		—			8		8		—			8
		$	674	$	(254	)	$	420	$	642	$	(169	)	$	473

(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):

2014	$	59
2015	$	52
2016	$	45
2017	$	42
2018	$	36