v2.4.0.6
INTANGIBLE ASSETS
 12 Months Ended
Dec. 31, 2011
INTANGIBLE ASSETS [Abstract]  
Intangible Assets
INTANGIBLE ASSETS
Intangible assets consisted of the following:
 
December 31, 2011
 
December 31, 2010
 
Cost
 
Accumulated
Amortization
 
Cost
 
Accumulated
Amortization
 
(Thousands of Dollars)
Customer relationships
$
81,025

 
$
(44,068
)
 
$
76,910

 
$
(35,983
)
Other
2,809

 
(843
)
 
2,809

 
(703
)
Total
$
83,834

 
$
(44,911
)
 
$
79,719

 
$
(36,686
)

All of our intangible assets are subject to amortization. Amortization expense for intangible assets was $8.3 million for the year ended December 31, 2011, and $7.6 million for each of the years ended December 31, 2010 and 2009. The estimated aggregate amortization expense for the next five years is as follows:
 
Amortization Expense
 
(Thousands of Dollars)
2012
$
8,213

2013
8,213

2014
8,213

2015
5,266

2016
2,319