
	Goodwill (Gross)		Accumulated Impairment Losses			Goodwill (Net)
January 1, 2019	$	243,053	$	(70,004	)	$	173,049
Business Combinations		11,332		—			11,332
Other		193		—			193

September 30, 2019	$	254,578	$	(70,004	)	$	184,574

Other changes included in the above table represent minor adjustments for the allocation of certain acquisitions still under measurement and four immaterial acquisitions completed during the nine months ended September 30, 2019.

We test goodwill for impairment annually during the fourth quarter of our fiscal year or earlier if there is an impairment indicator. No impairment was recognized during either of the nine month periods ended September 30, 2019 or 2018. Accumulated impairment losses included within the above table were incurred over multiple periods, with the latest impairment charge being recorded during the year ended December 31, 2010.

Intangibles, net

The following table provides the gross carrying amount, accumulated amortization and net book value for each major class of intangibles (in thousands):


	As of September 30, 2019						As of December 31, 2018
	Gross				Net		Gross				Net
	Carrying		Accumulated		Book		Carrying		Accumulated		Book
	Amount		Amortization		Value		Amount		Amortization		Value
Amortized intangibles:
Customer relationships	$	157,242	$	64,884	$	92,358	$	148,635	$	52,514	$	96,121
Covenants not-to-compete		16,035		9,857		6,178		14,682		7,572		7,110
Trademarks and trade names		67,048		21,489		45,559		64,432		18,256		46,176
Backlog		14,080		13,854		226		14,060		13,677		383

	$	254,405	$	110,084	$	144,321	$	241,809	$	92,019	$	149,790

The gross carrying amount of intangibles increased approximately $12.6 million during the nine months ended September 30, 2019 primarily due to business combinations. See Note 15, Business Combinations, for more information. Remaining estimated aggregate annual amortization expense is as follows (amounts, in thousands, are for the fiscal year ended):


Remainder of 2019	$	6,147
2020		23,867
2021		22,525
2022		21,604
2023		18,693
Thereafter		51,485