Software, net

We capitalize direct costs used in the development of internal-use software. Amounts capitalized are amortized over a period of two to ten years on a straight-line basis.

We purchase, license and incur costs to develop external use software to be used in the products we sell, lease or market to customers. Costs incurred in creating software are expensed when incurred as R&D until technological feasibility has been established, after which costs are capitalized up to the date the software is available for general release to customers. Generally, the software we develop reaches technological feasibility when a working model of the software is available. We capitalize the payments made for software that we purchase or license for use in our products that has previously met the technological feasibility criteria prior to our purchase or license. Amortization of capitalized software costs is recorded over the estimated economic life, which is typically eight to ten years.

For our game themes, we have determined that such products reach technological feasibility when internal testing is complete and the product is ready to be submitted to gaming regulators for approval. We incur and capitalize regulatory approval costs for our game themes after technological feasibility is achieved. Amortization of regulatory approval costs is recorded over the estimated economic life, which is typically two to four years.

Software, net consisted of the following:



	As of December 31,
	2018			2017
Software	$	1,101.6		$	1,003.2
Accumulated amortization	(816.3		)	(663.8		)
Software, net	$	285.3		$	339.4

In the years ended December 31, 2018 and 2017, we capitalized $108.5 million and $75.3 million, respectively, of software.

The following reflects amortization of software included within D&A:



	Year Ended December 31,
	2018		2017		2016
Amortization expense	$	160.5	$	153.3	$	158.9