The changes in the carrying amount of goodwill for the fiscal years ended December 31 were as follows:

	2020		2019
Balance as of January 1	$	719,934	$	715,644
Goodwill resulting from business combination - ITG		188,253		—
Goodwill resulting from business combination - DMS Disaster Consultants		—		(50	)
Goodwill resulting from business combination - We Are Vista		—		(370	)
Goodwill resulting from business combination - Olson ⁽¹⁾		—		3,047
Effect of foreign currency translation		1,726		1,663
Balance as of December 31	$	909,913	$	719,934

Intangible assets with definite lives are primarily amortized over periods ranging from approximately 1 to 10 years. The weighted-average period of amortization for all intangible assets, calculated as of December 31, 2020, is 8.5 years. The customer-related intangible assets, which consist of customer contracts, backlog, and non-contractual customer relationships, are being amortized based on estimated cash flows and respective estimated economic benefit of the assets. The weighted-average period of amortization of the customer-related intangibles calculated as of December 31, 2020 is 8.5 years. Intangible assets related to developed technology are being amortized on an accelerated basis over a weighted-average period, calculated as of December 31, 2020, of 4.8 years. Intangible assets with an indefinite life consist of a domain name.

	2020
	Gross Carrying Value		Accumulated Amortization			Net Carrying Value
Customer-related	$	142,849	$	(83,137	)	$	59,712
Developed technology		733		(653	)		80
Total amortizable intangible assets		143,582		(83,790	)		59,792
Intangible with indefinite life		95		—			95
Total other intangible assets	$	143,677	$	(83,790	)	$	59,887

	2019
	Gross Carrying Value		Accumulated Amortization			Net Carrying Value
Customer-related	$	95,038	$	(69,425	)	$	25,613
Developed technology		733		(612	)		121
Total amortizable intangible assets		95,771		(70,037	)		25,734
Intangible with indefinite life		95		—			95
Total other intangible assets	$	95,866	$	(70,037	)	$	25,829

Aggregate amortization expense for the years ended December 31, 2020, 2019, and 2018, was approximately $13.3 million, $8.1 million, and $10.0 million, respectively. The Company recognized impairment expense, included in indirect and selling expense, of $1.7 million in the second quarter of 2019 related to the intangible asset associated with a historical business acquisition. The estimated future amortization expense relating to intangible assets is as follows:

Year ending December 31,
2021	$	12,049
2022		11,727
2023		11,339
2024		10,856
2025		6,904
Thereafter		6,917
Total	$	59,792