INTANGIBLE ASSETS

12 Months Ended

Dec. 31, 2020

Intangible Assets, Net (Excluding Goodwill) [Abstract]

Intangible assets consisted of the following:


	Weighted-Average Amortization Period	December 31, 2020				December 31, 2019
		Cost		Accumulated Amortization		Cost		Accumulated Amortization
	(Years)	(Thousands of Dollars)
Customer contracts and relationships	18	$	863,900	$	(235,205)	$	863,900	$	(183,832)
Other	47	2,359		(845)		2,359		(795)
Total		$	866,259	$	(236,050)	$	866,259	$	(184,627)

Intangible assets are recorded at fair value as of the date acquired. All of our intangible assets are amortized on a straight-line basis. Amortization expense for intangible assets was $51.4 million for each of the years ended December 31, 2020, 2019 and 2018. The estimated aggregate amortization expense is $51.0 million for the years 2021 and 2022, $45.0 million for 2023 and 2024 and $38.0 million for 2025.