Goodwill and other intangible assets

6 Months Ended

Jun. 30, 2019

Goodwill and Intangible Assets Disclosure [Abstract]

The changes in the carrying amount of goodwill were as follows:



Six Months Ended June 30, 2019	Balance at January 1, 2019		Goodwill Acquired During the Year		Balance at June 30, 2019
	(In thousands)
Natural gas distribution	$	345,736	$	—	$	345,736
Construction materials and contracting	209,421		14,473		223,894
Construction services	109,765		—		109,765
Total	$	664,922	$	14,473	$	679,395



Six Months Ended June 30, 2018	Balance at January 1, 2018		Goodwill Acquired During the Year		Balance at June 30, 2018
	(In thousands)
Natural gas distribution	$	345,736	$	—	$	345,736
Construction materials and contracting	176,290		10,583		186,873
Construction services	109,765		—		109,765
Total	$	631,791	$	10,583	$	642,374



Year Ended December 31, 2018	Balance at January 1, 2018		Goodwill Acquired During the Year		Balance at December 31, 2018
	(In thousands)
Natural gas distribution	$	345,736	$	—	$	345,736
Construction materials and contracting	176,290		33,131		209,421
Construction services	109,765		—		109,765
Total	$	631,791	$	33,131	$	664,922

During 2019 and 2018, the Company completed one and four business combinations, respectively, and the results of the acquired businesses have been included in the Company's construction materials and contracting segment. These business combinations increased the construction materials and contracting segment's goodwill balance at June 30, 2019 and 2018, and December 31, 2018, as noted in the previous tables. As a result of the business combinations, other intangible assets increased $1.6 million at June 30, 2019, compared to December 31, 2018. For more information related to business combinations, see Note 9.

Other amortizable intangible assets were as follows:



	June 30, 2019		June 30, 2018		December 31, 2018
	(In thousands)
Customer relationships	$	14,601	$	15,587	$	22,720
Less accumulated amortization	5,629		13,191		13,535
	8,972		2,396		9,185
Noncompete agreements	3,179		2,546		2,605
Less accumulated amortization	1,798		1,877		1,956
	1,381		669		649
Other	6,578		6,458		6,458
Less accumulated amortization	5,608		5,333		5,477
	970		1,125		981
Total	$	11,323	$	4,190	$	10,815

Amortization expense for amortizable intangible assets for the three and six months ended June 30, 2019, was $500,000 and $1.1 million, respectively. Amortization expense for amortizable intangible assets for the three and six months ended June 30, 2018, was $300,000 and $700,000, respectively. Estimated amortization expense for identifiable intangible assets as of June 30, 2019, was:



	Remainder of 2019		2020		2021		2022		2023		Thereafter
	(In thousands)
Amortization expense	$	938	$	1,644	$	1,254	$	1,231	$	1,249	$	5,007