Note 8—Other Long-Term Assets, Net

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

	December 31,
	2015			2014
Intangible assets	$	610		$	644
Fair value of derivative instruments	9			27
Other	94			61

	713			732
Accumulated amortization	(327		)	(301		)

	$	386		$	431

Amortization expense for finite-lived intangible assets for the years ended December 31, 2015, 2014 and 2013 was $49 million, $57 million and $85 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, 2015							December 31, 2014
	Estimated Useful			Accumulated							Accumulated
	Lives (Years)	Cost		Amortization			Net		Cost		Amortization			Net
Customer contracts and relationships	1 – 20	$	537	$	(301	)	$	236	$	565	$	(277	)	$	288
Property tax abatement	7 – 13	38		(22		)	16		38		(18		)	20
Other agreements	25 – 70	28		(4		)	24		33		(4		)	29
Emission reduction credits (1)	N/A	7		—			7		8		—			8

		$	610	$	(327	)	$	283	$	644	$	(299	)	$	345

(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):

2016	$	44
2017	$	41
2018	$	36
2019	$	33
2020	$	31