GOODWILL AND OTHER INTANGIBLE ASSETS

The following table summarizes the carrying amount of goodwill by segment:



(in millions)	Environmental Solutions			Safety & Security Systems			Total
Balance at December 31, 2016	$	127.2		$	109.3		$	236.5
Acquisitions	136.0			—			136.0
Translation adjustments	0.4			4.4			4.8
Balance at December 31, 2017	263.6			113.7			377.3
Translation adjustments	(0.4		)	(1.8		)	(2.2		)
Balance at December 31, 2018	$	263.2		$	111.9		$	375.1

The following table summarizes the gross carrying amount and accumulated amortization of intangible assets for each major class of intangible assets:



	December 31, 2018							December 31, 2017
(in millions)	Gross Carrying Value		Accumulated Amortization			Net Carrying Value		Gross Carrying Value		Accumulated Amortization			Net Carrying Value
Definite-lived intangible assets:
Customer relationships^(a)	$	90.9	$	(12.1	)	$	78.8	$	90.9	$	(4.6	)	$	86.3
Other^(a)	2.9		(1.3		)	1.6		2.9		(0.9		)	2.0
Total definite-lived intangible assets	93.8		(13.4		)	80.4		93.8		(5.5		)	88.3
Indefinite-lived intangible assets:
Trade names	62.7		—			62.7		63.5		—			63.5
Total indefinite-lived intangible assets	62.7		—			62.7		63.5		—			63.5
Total intangible assets	$	156.5	$	(13.4	)	$	143.1	$	157.3	$	(5.5	)	$	151.8


(a)	Average useful life of customer relationships and other definite-lived intangible assets are estimated to be approximately 12 years and 7 years, respectively. The average useful life across all definite-lived intangible assets is estimated to be approximately 12 years.

Amortization expense for the years ended December 31, 2018 and 2017 was $8.0 million and $4.8 million, respectively. Amortization expense for the year ended December 31, 2016 was immaterial.

The Company currently estimates that aggregate amortization expense will be approximately $8.0 million in 2019, $8.0 million in 2020, $7.9 million in 2021, $7.8 million in 2022, $7.6 million in 2023 and $41.0 million thereafter. Actual amounts of amortization may differ from estimated amounts due to additional intangible asset acquisitions, changes in foreign currency rates, impairment of intangible assets and other events.