Intangible Assets

9 Months Ended

Sep. 30, 2013

Goodwill and Intangible Assets Disclosure [Abstract]

Intangible assets are included in other assets on our consolidated balance sheets. Intangible assets with definite lives are amortized. These assets are summarized as follows:



	As of September 30, 2013							As of December 31, 2012
(millions)	Gross Carrying Amount		Accumulated Amortization			Net		Gross Carrying Amount		Accumulated Amortization			Net
Intangible Assets with Definite Lives:
Customer relationships	$	70	$	(46	)	$	24	$	70	$	(41	)	$	29
Other	9		(7		)	2		9		(6		)	3
Total	$	79	$	(53	)	$	26	$	79	$	(47	)	$	32

Total amortization expense was $2 million and $6 million for the three and nine months ended September 30, 2013 and 2012, respectively. Estimated amortization expense for the remainder of 2013 and for future years is as follows:



(millions)	2013		2014		2015		2016		2017		2018 and thereafter
Estimated future amortization expense	$	2	$	7	$	7	$	7	$	2	$	1

Intangible assets with indefinite lives are not amortized. These assets are summarized as follows:



	As of September 30, 2013						As of December 31, 2012
(millions)	Gross Carrying Amount		Impairment Charges		Net		Gross Carrying Amount		Impairment Charges		Net
Intangible Assets with Indefinite Lives:
Trade names	$	22	$	—	$	22	$	22	$	—	$	22
Other	8		—		8		8		—		8
Total	$	30	$	—	$	30	$	30	$	—	$	30