Goodwill and other intangible assets

12 Months Ended

Dec. 31, 2018

Goodwill and Intangible Assets Disclosure [Abstract]

Goodwill and Other Intangible Assets

The changes in the carrying amount of goodwill for the year ended December 31, 2018, were as follows:



	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

The changes in the carrying amount of goodwill for the year ended December 31, 2017, were as follows:



	Balance at January 1, 2017		Goodwill Acquired During the Year		Balance at December 31, 2017
	(In thousands)
Natural gas distribution	$	345,736	$	—	$	345,736
Construction materials and contracting	176,290		—		176,290
Construction services	109,765		—		109,765
Total	$	631,791	$	—	$	631,791

During 2018, the Company completed four acquisitions and the results of the acquired businesses have been included in the Company's construction materials and contracting segment. At December 31, 2018, the construction materials and contracting segment's goodwill increased by $33.1 million and other intangible assets increased by $8.2 million for these acquisitions. For more information about these acquisitions, see Note 3.

Other amortizable intangible assets at December 31 were as follows:



	2018		2017
	(In thousands)
Customer relationships	$	22,720	$	15,248
Less accumulated amortization	13,535		13,382
	9,185		1,866
Noncompete agreements	2,605		2,430
Less accumulated amortization	1,956		1,805
	649		625
Other	6,458		6,990
Less accumulated amortization	5,477		5,644
	981		1,346
Total	$	10,815	$	3,837

Amortization expense for amortizable intangible assets for the years ended December 31, 2018, 2017 and 2016, was $1.2 million, $2.0 million and $2.5 million, respectively. The amounts of estimated amortization expense for identifiable intangible assets as of December 31, 2018, were:



	2019		2020		2021		2022		2023		Thereafter
	(In thousands)
Amortization expense	$	1,856	$	1,486	$	1,096	$	1,072	$	1,006	$	4,299