Property, Plant and Equipment

12 Months Ended

Dec. 31, 2017

Property, Plant and Equipment [Abstract]

The following is a summary of property, plant and equipment, at cost less accumulated depreciation and amortization as of December 31:



	Estimated Useful Life (years)	2017		2016
Land and land improvements	0-15	$	16.0	$	16.9
Buildings and building improvements	15-30	112.9		129.8
Machinery, tools and equipment	5-12	108.2		121.0
Leasehold improvements ⁽¹⁾	10	28.3		29.4
Computer equipment	3	153.8		133.8
Computer software	5-10	146.6		224.7
Furniture and fixtures	5-8	73.4		75.0
Tooling	3-5	136.4		123.1
Construction in progress		7.7		10.3
Total property plant and equipment, at cost		$	783.3	$	864.0
Less accumulated depreciation and amortization		418.8		477.0
Total property plant and equipment, net		$	364.5	$	387.0


⁽¹⁾	The estimated useful life for leasehold improvements is the lesser of 10 years or the term of the lease.

During 2017, 2016 and 2015, depreciation expense, computed on a straight-line basis over the estimated useful lives of the related assets, was $92.9, $61.8 and $40.7, respectively. The decrease in computer software and accumulated depreciation and amortization is primarily related to the write-off of certain fully depreciated enterprise resource planning assets.