	June 30, 2021					December 31, 2020
	Gross Carrying Amount		Accumulated Amortization			Gross Carrying Amount		Accumulated Amortization
Customer relationships	$	1,534,268	$	(205,237	)	$	195,435	$	(94,690	)
Trade names		185,361		(95,783	)		52,061		(38,138	)
Subcontractor relationships		5,440		(2,851	)		5,440		(1,944	)
Non-compete agreements		3,719		(2,268	)		3,719		(2,001	)
Total intangible assets	$	1,728,788	$	(306,139	)	$	256,655	$	(136,773	)

In connection with the BMC Merger and John’s Lumber acquisition, we recorded intangible assets of $1,472.6 million, which includes $1,339.3 million of customer relationships and $133.3 million of trade names. The weighted average useful lives of the acquired intangible assets are 11.3 years in total, 11.4 years for customer relationships and 5.5 years for trade names, respectively. The fair value of acquired intangible assets was estimated by applying an income approach. That measure is 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 disposal of our gypsum stand-alone operations, we reclassified $0.5 million of intangible assets to held-for-sale assets as of June 30, 2021. Refer to Note 2 – Business Combinations and Dispositions for additional information.

During the three and six months ended June 30, 2021, we recorded amortization expense in relation to the above-listed intangible assets of $80.6 million and $169.3 million, respectively. During the three and six months ended June 30, 2020, we recorded amortization expense in relation to the above-listed intangible assets of $5.6 million and $11.3 million, respectively.

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

2021 (from July 1, 2021)	$	168,905
2022		200,574
2023		174,217
2024		153,359
2025		134,723
Thereafter		590,871
Total future net intangible amortization expense	$	1,422,649