5. GOODWILL AND INTANGIBLE ASSETS

Goodwill The following table provides a reconciliation of changes in goodwill for the nine months ended September 30, 2018:



	Driveline			Metal Forming			Powertrain			Casting		Consolidated
	(in millions)
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
Sale of business	—			—			(15.1		)	—		(15.1		)
Foreign currency translation	(0.3		)	(5.6		)	(2.5		)	—		(8.4		)
Balance as of September 30, 2018	$	212.1		$	554.2		$	461.2		$	405.5	$	1,633.0

In April 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.

Intangible Assets 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:



	September 30,							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	$	37.4	$	(18.6	)	$	18.8	$	35.6	$	(14.3	)	$	21.3
e-AAM in-process research and development	5.4		(0.6		)	4.8		5.9		—			5.9
Customer platforms	952.2		(105.8		)	846.4		952.2		(52.9		)	899.3
Customer relationships	147.0		(14.2		)	132.8		151.8		(7.3		)	144.5
Technology and other	150.8		(18.4		)	132.4		150.8		(9.3		)	141.5
Total	$	1,292.8	$	(157.6	)	$	1,135.2	$	1,296.3	$	(83.8	)	$	1,212.5

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 the second quarter 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 $24.8 million and $74.5 million for the three and nine months ended September 30, 2018 respectively, and $24.4 million and $50.8 million for the three and nine months ended September 30, 2017, respectively. Estimated amortization expense for each of the years 2018 through 2022 is approximately $100 million.