4. Long-Lived Assets

Property, plant and equipment consisted of the following (dollars in thousands):


	March 31,		December 31,
	2018		2017
Property, plant and equipment cost	$	1,151,184	$	1,131,271
Less accumulated depreciation		(544,097)		(522,973)
	$	607,087	$	608,298

Depreciation expense amounted to $23.0 million and $20.8 million for the three months ended March 31, 2018 and 2017, respectively.

The gross carrying amount and accumulated amortization of identifiable intangible assets are as follows (dollars in thousands):


	March 31, 2018						December 31, 2017
	Gross				Net		Gross				Net
	Carrying		Accumulated		Carrying		Carrying		Accumulated		Carrying
	Value		Amortization		Value		Value		Amortization		Value
Subject to amortization:
Customer relationships	$	21,709	$	18,310	$	3,399	$	21,709	$	17,983	$	3,726
Trade name and other		320		320		—		320		320		—
		22,029		18,630		3,399		22,029		18,303		3,726
Not subject to amortization:
Brand name		27,300		—		27,300		27,300		—		27,300
		27,300		—		27,300		27,300		—		27,300
	$	49,329	$	18,630	$	30,699	$	49,329	$	18,303	$	31,026

As of March 31, 2018, the Company had goodwill of $12.1 million. As further discussed in Note 14, the Company has concluded it has only one segment and one reporting unit. As such, the Company classifies all goodwill in the one segment.

Amortization expense amounted to $0.3 million and $0.4 million for the three months ended March 31, 2018 and 2017, respectively. Estimated amortization expense for the next five years and thereafter is as follows (dollars in thousands):


Year ended December 31,
2018 (remaining months)	$	980
2019		930
2020		574
2021		270
2022		223
Thereafter		422
	$	3,399

dfjld