	2021					2020
	Gross Carrying Amount		Accumulated Amortization			Gross Carrying Amount		Accumulated Amortization
	(in thousands)
Customer relationships	$	1,781,264	$	(328,540	)	$	195,435	$	(94,690	)
Trade names		201,861		(155,141	)		52,061		(38,138	)
Subcontractor relationships		5,440		(3,757	)		5,440		(1,944	)
Non-compete agreements		13,519		(3,243	)		3,719		(2,001	)
Developed technology		95,600		(3,594	)		-		-
Total intangible assets	$	2,097,684	$	(494,275	)	$	256,655	$	(136,773	)

During the years ended December 31, 2021, 2020, and 2019, we recorded amortization expense in relation to the above-listed intangible assets of $358.1 million, $22.1 million, and $16.1 million, respectively. We recorded no intangible asset impairment charges for the years ended December 31, 2021, 2020 or 2019.

In connection with the BMC Merger and current year acquisitions, we recorded intangible assets of $1.8 billion, which includes $1.6 billion of customer relationships, $149.8 million of trade names, $95.6 million of developed technology, and $9.8 million of non-compete agreements. The weighted average useful lives of the acquired intangible assets are 12.2 years in total, 12.5 years for customer relationships, 7.0 years for trade names, 9.1 years for developed technology and 5.7 years for non-compete agreements. The fair value of acquired customer relationship intangible assets was primarily estimated by applying the multi-period excess earnings method, which involved the use of significant estimates and assumptions primarily related to forecasted revenue growth rates, gross margin, contributory asset charges, customer attrition rates, and market-participant discount rates. These measures are based on significant Level 3 inputs not observable in the market. Key assumptions developed based on the Company’s historical experience, future projections and comparable market data include future cash flows, long-term growth rates, attrition rates and discount rates.

In connection with the Gypsum Divestiture, we derecognized $0.5 million of intangible assets, net. Refer to Note 3 – Business Combinations and Dispositions for additional information.

The following table presents the estimated amortization expense for intangible assets for the years ending December 31 (in thousands):

2022	$	260,867
2023		230,168
2024		201,423
2025		172,843
2026		153,052
Thereafter		585,056
Total future net intangible amortization expense	$	1,603,409