Intangible Assets

12 Months Ended

Dec. 31, 2014

Goodwill and Intangible Assets Disclosure [Abstract]

Intangible Assets

Intangible Assets

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

	As of December 31, 2014							As of December 31, 2013
(millions)	Gross Carrying Amount		Accumulated Amortization			Net		Gross Carrying Amount		Accumulated Amortization			Net
Intangible Assets with Definite Lives:
Customer relationships	$	70	$	(54	)	$	16	$	70	$	(48	)	$	22
Other	9		(7		)	2		9		(6		)	3
Total	$	79	$	(61	)	$	18	$	79	$	(54	)	$	25

The weighted average amortization periods are 10 years for customer relationships and 11 years for other intangible assets with definite lives. Total amortization expense was $7 million in 2014, $7 million in 2013 and $8 million in 2012. Estimated annual amortization expense is as follows:

(millions)	2015		2016		2017		2018		2019 and thereafter
Estimated annual amortization expense	$	7	$	7	$	2	$	1	$	1

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

	As of December 31, 2014						As of December 31, 2013
(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	9		1		8		8		1		7
Total	$	31	$	1	$	30	$	30	$	1	$	29

In 2014, 2013 and 2012, there was no impairment for any of our customer relationship or trade name intangible assets.

Intangible assets are included in other assets on our consolidated balance sheets, except for approximately $5 million of other indefinite-lived intangible assets which met the criteria to be classified as held for sale during the second quarter of 2014 and therefore are included in other current assets on our consolidated balance sheet as of December 31, 2014.