INTANGIBLE ASSETS

Intangible assets consisted of the following:



	December 31, 2013					December 31, 2012
	Cost		Accumulated Amortization			Cost		Accumulated Amortization
	(Thousands of Dollars)
Customer relationships	$	127,614	$	(58,230	)	$	137,470	$	(46,951	)
Other	2,359		(494		)	2,359		(443		)
Total	$	129,973	$	(58,724	)	$	139,829	$	(47,394	)

All of our intangible assets are subject to amortization. Amortization expense for intangible assets was $13.8 million, $7.8 million and $8.3 million for the years ended December 31, 2013, 2012 and 2011, respectively. The estimated aggregate amortization expense for the next five years is as follows:



	Amortization Expense
	(Thousands of Dollars)
2014	$	12,579
2015	$	9,709
2016	$	6,840
2017	$	6,840
2018	$	6,840