Goodwill and other intangible assets (Tables)

12 Months Ended

Dec. 31, 2018

Goodwill and Intangible Assets Disclosure [Abstract]

Changes in the carrying amount of goodwill

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

Other amortizable intangible assets

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

Future Amortization Expense

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