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Property, Plant, and Equipment
12 Months Ended
Dec. 31, 2015
Property, Plant and Equipment [Abstract]  
Property, Plant, and Equipment
Property, Plant, and Equipment
Property, plant, and equipment on the Consolidated Balance Sheets is summarized as follows:
 
 
December 31,
(Thousands)
 
2015
 
2014
Land
 
$
7,457

 
$
7,458

Buildings
 
133,427

 
129,673

Machinery and equipment
 
597,423

 
579,906

Software
 
38,505

 
34,525

Construction in progress
 
15,449

 
16,818

Allowances for depreciation
 
(565,203
)
 
(534,908
)
Subtotal
 
227,058

 
233,472

Capital leases
 
10,913

 
10,912

Allowances for depreciation
 
(2,242
)
 
(1,988
)
Subtotal
 
8,671

 
8,924

Mineral resources
 
4,979

 
4,980

Mine development
 
25,681

 
16,399

Allowances for amortization and depletion
 
(2,760
)
 
(16,187
)
Subtotal
 
27,900

 
5,192

Property, plant, and equipment — net
 
$
263,629

 
$
247,588


The Company received $63.5 million from the U.S. Department of Defense (DoD) in previous periods for reimbursement of the DoD's share of the cost of the equipment in property, plant, and equipment and the reimbursements as unearned income, a liability on the Consolidated Balance Sheets. The equipment was placed in service during 2012, and its full cost is being depreciated in accordance with Company policy. The unearned income liability is being reduced ratably with the depreciation expense recorded over the life of the equipment.
Unearned income was reduced by $5.8 million and $4.7 million in 2015 and 2014, respectively, and credited to cost of sales in the Consolidated Statements of Income, offsetting the impact of the depreciation expense on the associated equipment on the Company's cost of sales and gross margin.
We recorded depreciation and depletion expense of $32.8 million in 2015, $37.5 million in 2014, and $36.1 million in 2013. The expense is net of the above-referenced reductions in the unearned income liability. Depreciation, depletion, and amortization as shown on the Consolidated Statement of Cash Flows is also net of the reduction in the unearned income liability in 2015, 2014, and 2013. The net book value of capitalized software was $9.5 million and $6.1 million at December 31, 2015 and December 31, 2014, respectively. Depreciation expense related to software was $2.3 million in 2015, $1.8 million in 2014, and $1.7 million in 2013.