Intangible Assets

The gross carrying amount and accumulated amortization for intangible assets consisted of the following ($ in millions):



	September 30, 2018
	Gross Carrying Amount		Accumulated Amortization		Accumulated Impairments		Accumulated Foreign Exchange			Net Carrying Amount
Amortized Intangible Assets
Advanced Materials & Structures
Customer relationships	$	276.3	$	46.0	$	—	$	(0.2	)	$	230.5
Developed technology	34.0		8.0		—		0.1			25.9
Trade names	21.8		0.8		20.7		0.3			—
Non-compete agreements	2.9		1.6		—		—			1.3
Patents	1.5		0.3		—		—			1.2
Total	$	336.5	$	56.7	$	20.7	$	0.2		$	258.9

Unamortized Intangible Assets (Advanced Materials & Structures)
Trade names	$	20.0	$	—	$	0.1	$	(0.2	)	$	20.1



	December 31, 2017
	Gross Carrying Amount		Accumulated Amortization		Accumulated Impairments		Accumulated Foreign Exchange			Net Carrying Amount
Amortized Intangible Assets
Advanced Materials & Structures
Customer relationships	$	276.3	$	32.9	$	—	$	(2.3	)	$	245.7
Developed technology	34.0		6.0		—		(0.1		)	28.1
Customer contracts	0.9		0.9		—		—			—
Trade names	21.8		0.8		20.7		0.3			—
Non-compete agreements	2.9		1.1		—		—			1.8
Patents	1.5		0.3		—		—			1.2
Total	$	337.4	$	42.0	$	20.7	$	(2.1	)	$	276.8

Unamortized Intangible Assets (Advanced Materials & Structures)
Trade names	$	20.0	$	—	$	—	$	(0.4	)	$	20.4

Amortization expense of intangible assets was $5.2 million and $5.2 million for the three months ended September 30, 2018 and 2017, respectively, and $15.6 million and $15.7 million for the nine months ended September 30, 2018 and 2017, respectively. Finite-lived intangibles in the AMS segment are expensed using the straight-line amortization method. The estimated average aggregate amortization expense is $20.3 million in each of the next five years.