Note 5. Goodwill and Intangible Assets

The following table presents changes in goodwill, all of which has been allocated to the GDSO segment (in thousands):


	Total
Balance at December 31, 2018	$	327,406
Dispositions (1)		(2,932)
Balance at December 31, 2019	$	324,474

(1)

Dispositions represent derecognition of goodwill associated with the sale and disposition of certain assets (see Note 7).

Intangible assets consisted of the following (in thousands):


	Gross				Net
	Carrying		Accumulated		Intangible		Amortization
	Amount		Amortization		Assets		Period
At December 31, 2019
Intangible assets subject to amortization:
Terminalling services	$	26,365	$	(16,429)	$	9,936	20 years
Customer relationships		43,986		(41,630)		2,356	2-15 years
Supply contracts		87,578		(54,655)		32,923	5-15 years
Other intangible assets		5,195		(3,645)		1,550	3-20 years
Total intangible assets	$	163,124	$	(116,359)	$	46,765
At December 31, 2018
Intangible assets subject to amortization:
Terminalling services	$	26,365	$	(15,093)	$	11,272	20 years
Customer relationships		43,986		(41,195)		2,791	2-15 years
Supply contracts		87,578		(46,430)		41,148	5-15 years
Other intangible assets		8,575		(5,254)		3,321	1-20 years
Total intangible assets	$	166,504	$	(107,972)	$	58,532

The aggregate amortization expense was approximately $11.4 million, $11.0 million and $9.2 million for the years ended December 31, 2019, 2018 and 2017, respectively. In addition, the Partnership recognized amortization expense related to leasehold interests of $0, $0.3 million and $0.6 million in 2019, 2018 and 2017, respectively.

The estimated annual intangible asset amortization expense for future years ending December 31 is as follows (in thousands):


2020	$	10,839
2021		10,434
2022		7,199
2023		6,768
2024		5,103
Thereafter		6,422
Total intangible assets	$	46,765