5. GOODWILL AND OTHER INTANGIBLE ASSETS

Goodwill The following table provides a reconciliation of changes in goodwill for the year ended December 31, 2018 and the year ended December 31, 2017:



	Driveline			Metal Forming			Powertrain			Casting			Consolidated
	(in millions)
Balance as of January 1, 2017	$	130.1		$	23.9		$	—		$	—		$	154.0
Acquisition of MPG	—			515.3			471.6			405.5			1,392.4
Acquisition of USM Mexico	80.4			—			—			—			80.4
Foreign currency translation	0.6			19.7			7.2			—			27.5
Balance as of December 31, 2017	$	211.1		$	558.9		$	478.8		$	405.5		$	1,654.3
Acquisition of MPG	—			0.9			—			—			0.9
Acquisition of USM Mexico	1.3			—			—			—			1.3
Impairment charge	—			—			(80.0		)	(405.5		)	(485.5		)
Sale of business	—			—			(15.1		)	—			(15.1		)
Foreign currency translation	(0.3		)	(7.4		)	(6.4		)	—			(14.1		)
Balance as of December 31, 2018	$	212.1		$	552.4		$	377.3		$	—		$	1,141.8

We conduct our annual goodwill impairment test in the fourth quarter of each year. In performing this test, we utilize a third-party valuation specialist to assist management in determining the fair value of our reporting units. Fair value of each reporting unit is estimated based on a combination of discounted cash flows and the use of pricing multiples derived from an analysis of comparable public companies multiplied against historical and/or anticipated financial metrics of each reporting unit. These calculations contain uncertainties as they require management to make assumptions including, but not limited to, market comparables, future cash flows of the reporting units, and appropriate discount and long-term growth rates. This fair value determination is categorized as Level 3 within the fair value hierarchy.

As a result of our test in the fourth quarter of 2018, we determined that the carrying values of our Powertrain and Casting reporting units were greater than their respective fair values. As such, we recorded non-cash goodwill impairment charges of $80.0 million associated with Powertrain and $405.5 million associated with Casting in 2018. These impairments were primarily the result of a general contraction of pricing multiples associated with capital intensive businesses such as the business conducted by our Powertrain and Casting reporting units, as well as a decline in the projected cash flows of these reporting units under our long-range plan completed in the fourth quarter of 2018, as compared to the long-range plan completed in the fourth quarter of 2017.

The decline in projected cash flows for the Powertrain reporting unit was primarily the result of decreased contribution margin on lower production volumes for certain passenger car programs that we support. The decline in projected cash flows for the Casting reporting unit was primarily the result of a projected increase in labor costs in an effort to address workforce shortages at certain locations, as well as an increase in other maintenance and capital requirements. At December 31, 2018, accumulated goodwill impairment losses were $485.5 million and there were no such accumulated goodwill impairment losses as of December 31, 2017.

In 2019, we plan to initiate a new global restructuring program to further streamline our business by consolidating our four existing business units into three business units. This activity will occur through the disaggregation of our Powertrain business unit, with a portion moving into our Driveline business unit and a portion moving into our Metal Forming business unit. As a result of this activity, we expect to evaluate goodwill for impairment in the first quarter of 2019.

These goodwill impairment charges represented a triggering event for testing the recoverability of other long-lived assets, including property, plant and equipment and amortizable intangible assets associated with our Powertrain and Casting segments. No further impairments were identified.

In the second quarter of 2018, we completed the sale of the aftermarket business associated with our Powertrain segment. We allocated $15.1 million of goodwill to the sold business, which represents the fair value of the business sold relative to the fair value of the associated reporting unit.

Other Intangible Assets As a result of our acquisitions of MPG and USM Mexico, AAM identified and recognized $1,254.8 million of intangible assets that are subject to amortization. These intangible assets were assigned useful lives ranging from five to 17 years and the weighted-average amortization period for all intangible assets recognized as a result of these acquisitions is 13.6 years from the dates of the acquisitions. The intangible assets were valued using primarily the relief from royalty method or the multi-period excess earnings method, both of which utilize significant unobservable inputs. These inputs are defined in the fair value hierarchy as Level 3 inputs, which require management to make estimates and assumptions regarding certain financial measures using forecasted or projected information.

The following table provides a reconciliation of the gross carrying amount and associated accumulated amortization for AAM's total intangible assets, which are all subject to amortization, as of December 31, 2018 and December 31, 2017:



	December 31,							December 31,
	2018							2017
	Gross Carrying Amount		Accumulated Amortization			Net Carrying Amount		Gross Carrying Amount		Accumulated Amortization			Net Carrying Amount
	(in millions)
Capitalized computer software	$	38.0	$	(20.1	)	$	17.9	$	35.6	$	(14.3	)	$	21.3
Customer platforms	952.2		(123.5		)	828.7		952.2		(52.9		)	899.3
Customer relationships	147.0		(16.5		)	130.5		151.8		(7.3		)	144.5
Technology and other	156.2		(22.2		)	134.0		150.8		(9.3		)	141.5
Total	$	1,293.4	$	(182.3	)	$	1,111.1	$	1,290.4	$	(83.8	)	$	1,206.6

As a result of the acquisition of MPG in 2017, we recorded intangible assets related to aftermarket customer relationships that were associated with the Powertrain aftermarket business that we sold in the second quarter of 2018. As such, during 2018 we reduced the gross carrying amount of our customer relationships by $4.8 million, and reduced the associated accumulated amortization by $0.3 million.

Amortization expense for our intangible assets was $99.4 million for the year ended December 31, 2018, $75.3 million for the year ended December 31, 2017, and $5.0 million for the year ended December 31, 2016. The increase in amortization expense in 2018, as compared to 2017, was primarily attributable to the impact of twelve months of amortization on the MPG intangibles in 2018, as compared to nine months of amortization in 2017. The increase in amortization expense in 2017, as compared to 2016, was attributable to the increase in intangible assets as a result of the MPG and USM Mexico acquisitions in 2017. Estimated amortization expense is approximately $100 million per year for each of the years 2019 through 2023.