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Property, Plant and Equipment
12 Months Ended
Dec. 31, 2017
Property, Plant and Equipment [Abstract]  
Property, Plant and Equipment
Property, Plant and Equipment

Property, plant and equipment consisted of the following at December 31, 2017 and 2016:
 
 
Estimated
 
December 31,
 
December 31,
 
Useful Lives
 
2017
 
2016
 
  
 
(in thousands)
Land
 
 
$
6,418

 
$
13,057

Buildings and structures
10 – 40 years
 
195,540

 
115,960

Cable and wire
4 – 40 years
 
286,999

 
235,471

Equipment and software
2 – 17 years
 
730,228

 
720,830

Plant in service
 
 
1,219,185

 
1,085,318

Plant under construction
 
 
62,202

 
73,759

Total property, plant and equipment
 
 
1,281,387

 
1,159,077

Less accumulated amortization and depreciation
 
 
595,060

 
460,955

Property, plant and equipment, net
 
 
$
686,327

 
$
698,122



Depreciation expense for the years ended December 31, 2017, 2016, and 2015, was $151.1 million, $124.0 million, and $69.3 million, respectively. The Company leases fiber under indefeasible right of use agreements, (IRUs). The value of the Company's IRUs, which are classified as capital lease agreements that were included in property, plant and equipment, net as of December 31, 2017 was $5.9 million.

At December 31, 2017 and 2016, the Company had unamortized capitalized software costs for software in service of $28.0 million and $15.7 million, respectively.

At December 31, 2017 and 2016, plant under construction consisted primarily of equipment and software, which was not placed into service.