4. Long-Lived Assets

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


	March 31,		December 31,
	2016		2015
Property, plant and equipment cost	$	961,009	$	937,927
Less accumulated depreciation		378,738		358,820
	$	582,271	$	579,107

Depreciation expense amounted to $21.4 million and $20.7 million for the three months ended March 31, 2016 and 2015, respectively.

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


	March 31, 2016						December 31, 2015
	Gross				Net		Gross				Net
	Carrying		Accumulated		Carrying		Carrying		Accumulated		Carrying
	Value		Amortization		Value		Value		Amortization		Value
Subject to amortization:
Customer relationships	$	21,709	$	14,753	$	6,956	$	21,709	$	14,238	$	7,471
Trade name and other		320		272		48		320		263		57
		22,029		15,025		7,004		22,029		14,501		7,528
Not subject to amortization:
Brand name		27,300		—		27,300		27,300		—		27,300
		27,300		—		27,300		27,300		—		27,300
	$	49,329	$	15,025	$	34,304	$	49,329	$	14,501	$	34,828

Amortization expense amounted to $0.5 million and $0.6 million for the three months ended March 31, 2016 and 2015, respectively. Estimated amortization expense for the next five years and thereafter is as follows (dollars in thousands):


Year ended December 31,
2016 (remaining months)	$	1,576
2017		1,703
2018		1,307
2019		930
2020		574
Thereafter		914
	$	7,004