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Intangible Assets | 12 Months Ended
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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 |

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- Definition The entire disclosure for all or part of the information related to intangible assets. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 142 -Paragraph 44, 45, 46 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=7658586&loc=d3e16323-109275 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=7658586&loc=d3e16373-109275 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=7658586&loc=d3e16265-109275 + Details
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