Intangible Assets

12 Months Ended

Dec. 31, 2011

Intangible Assets [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. Amortization expense for intangible assets was $18 million, $22 million, and $25 million for the years ended December 31, 2011, 2010, and 2009, respectively. The estimated aggregate amortization expense is expected to be $20 million for each of the next five years.