4. Long-Lived Assets

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


	June 30,		December 31,
	2016		2015
Property, plant and equipment cost	$	989,256	$	937,927
Less accumulated depreciation		399,705		358,820
	$	589,551	$	579,107

Depreciation expense amounted to $22.0 million and $43.3 million for the three and six months ended June 30, 2016, respectively. Depreciation expense amounted to $21.3 million and $42.0 million for the three and six months ended June 30, 2015, respectively.

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


	June 30, 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	$	15,268	$	6,441	$	21,709	$	14,238	$	7,471
Trade name and other		320		282		38		320		263		57
		22,029		15,550		6,479		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	$	15,550	$	33,779	$	49,329	$	14,501	$	34,828

Amortization expense amounted to $0.5 million and $1.1 million for the three and six months ended June 30, 2016, respectively. Amortization expense amounted to $0.6 million and $1.2 million for the three and six months ended June 30, 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)	$	1,051
2017		1,703
2018		1,307
2019		930
2020		574
Thereafter		914
	$	6,479