INTANGIBLE ASSETS

Intangible assets consisted of the following:



		December 31, 2017					December 31, 2016
	Weighted-Average Amortization Period	Cost		Accumulated Amortization			Cost		Accumulated Amortization
	(Years)	(Thousands of Dollars)
Customer contracts and relationships	18	$	863,950	$	(81,136	)	$	166,950	$	(41,582	)
Other	47	2,359		(694		)	2,359		(644		)
Total		$	866,309	$	(81,830	)	$	169,309	$	(42,226	)

All of our intangible assets are amortized on a straight-line basis. Amortization expense for intangible assets was $39.6 million, $13.9 million and $16.7 million for the years ended December 31, 2017, 2016 and 2015, respectively. The estimated aggregate amortization expense is approximately $51.0 million for each of the years 2018 through 2022.