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Intangible Assets
 12 Months Ended
Dec. 31, 2012
Intangible Assets, Net (Excluding Goodwill) [Abstract]  
INTANGIBLE ASSETS
8.
INTANGIBLE ASSETS
Intangible assets include trade names, customer lists, air emission credits, and various other agreements. All of our intangible assets are subject to amortization. Intangible assets with finite useful lives are amortized on a straight-line basis. The gross carrying amount of intangible assets as of December 31, 2012 and 2011 was $429 million and $417 million, respectively. The accumulated amortization of intangible assets was $216 million and $190 million as of December 31, 2012 and 2011, respectively. Amortization expense for intangible assets was $22 million, $18 million, and $22 million for the years ended December 31, 2012, 2011, and 2010, respectively. The estimated aggregate amortization expense for the years ending December 31, 2013 through December 31, 2017 is as follows (in millions):
 
Amortization
Expense
2013
$
21

2014
21

2015
21

2016
18

2017
7