Property, Plant and Equipment

12 Months Ended

Dec. 31, 2018

Property, Plant and Equipment [Abstract]

Property, plant and equipment consist of the following at December 31:



	2018			2017
Land	$	4,027		$	4,027
Building and improvements	92,470			91,766
Machinery and equipment	227,795			219,675
Construction in progress	8,043			7,837
	332,335			323,305
Less: Accumulated depreciation	(219,090		)	(207,076		)
	$	113,245		$	116,229

Internal-use software, included in gross machinery and equipment at December 31, 2018 and 2017 was $47.8 million and $45.3 million, respectively, with related accumulated depreciation of $34.5 million and $30.4 million, respectively. Internal use software depreciation expense was $4.7 million, $4.5 million and $4.0 million for the years ended December 31, 2018, 2017 and 2016, respectively.

We lease various manufacturing facilities, office facilities and equipment under operating and capital leases. Leasehold improvements related to these facilities are included in building and improvements above. Rental expense on operating leases was approximately $8.7 million, $6.5 million and $6.0 million for the years ended December 31, 2018, 2017 and 2016, respectively. During 2018 and 2017, we entered into capital lease obligations of $0.2 million and $0.8 million in connection with the purchase of equipment.

The aggregate future minimum lease commitments for leases at December 31, 2018 are as follows:



	Operating Leases		Capital Leases
2019	$	8,759	$	347
2020	5,520		217
2021	2,929		21
2022	2,406		75
2023	1,563		—
Thereafter	1,497		—