	September 30, 2021					December 31, 2020
	Gross Carrying Amount		Accumulated Amortization			Gross Carrying Amount		Accumulated Amortization
Customer relationships	$	1,709,115	$	(266,507	)	$	195,435	$	(94,690	)
Trade names		201,861		(125,375	)		52,061		(38,138	)
Subcontractor relationships		5,440		(2,360	)		5,440		(1,944	)
Non-compete agreements		13,519		(2,586	)		3,719		(2,001	)
Developed technology		97,200		(944	)		-		-
Total intangible assets	$	2,027,135	$	(397,772	)	$	256,655	$	(136,773	)

In connection with the BMC Merger and current year acquisitions, we recorded intangible assets of $1.8 billion, which includes $1.5 billion of customer relationships, $149.8 million of trade names, $97.2 million of developed technology, and $9.8 million of non-compete agreements. The weighted average useful lives of the acquired intangible assets are 11.1 years in total, 11.3 years for customer relationships, 7.0 years for trade names, 9.0 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 2 – Business Combinations and Dispositions for additional information.

During the three and nine months ended September 30, 2021, we recorded amortization expense in relation to the above-listed intangible assets of $92.3 million and $261.6 million, respectively. During the three and nine months ended September 30, 2020, we recorded amortization expense in relation to the above-listed intangible assets of $5.4 million and $16.7 million, respectively.

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

2021 (from October 1, 2021)	$	96,494
2022		248,732
2023		217,259
2024		187,624
2025		165,262
Thereafter		713,992
Total future net intangible amortization expense	$	1,629,363