Ashland reviews goodwill and indefinite-lived intangible assets for impairment annually or when events and circumstances indicate an impairment may have occurred. This annual assessment is performed as of July 1 and consists of Ashland determining each reporting unit’s current fair value compared to its current carrying value. For its July 1, 2018 assessment, Ashland determined that its reporting units for the allocation of goodwill were its three reportable segments: Specialty Ingredients, Composites and Intermediates and Solvents. At that time, Ashland determined no additional impairment existed. Due to the expected divestiture of the Composites reporting unit, Ashland has two remaining reporting units as of June 30, 2019: Specialty Ingredients and Intermediates and Solvents. See Note B for more information on the expected divestiture.

The following is a progression of goodwill by reportable segment for the nine months ended June 30, 2019.

	Specialty			Intermediates
(In millions)	Ingredients			and Solvents		(a)	Total
Balance at September 30, 2018	$	2,304		$	—		$	2,304
Acquisitions		—			—			—
Currency translation		(19	)		—			(19	)
Balance at June 30, 2019	$	2,285		$	—		$	2,285

Intangible assets principally consist of trademarks and trade names, intellectual property and customer and supplier relationships. Intangible assets classified as finite are amortized on a straight-line basis over their estimated useful lives. The cost of trademarks and trade names is amortized principally over 3 to 25 years, intellectual property over 5 to 25 years, and customer and supplier relationships over 3 to 24 years.

Ashland annually reviews indefinite-lived intangible assets for possible impairment or whenever events or changes in circumstances indicate that carrying amounts may not be recoverable.

Intangible assets were comprised of the following as of June 30, 2019 and September 30, 2018.

	June 30, 2019
	Gross					Net
	carrying		Accumulated			carrying
(In millions)	amount		amortization			amount
Definite-lived intangibles
Trademarks and trade names	$	66	$	(28	)	$	38
Intellectual property		719		(382	)		337
Customer and supplier relationships		753		(288	)		465
Total definite-lived intangibles		1,538		(698	)		840

Indefinite-lived intangibles
Trademarks and trade names		278		—			278
Total intangible assets	$	1,816	$	(698	)	$	1,118

	September 30, 2018
	Gross					Net
	carrying		Accumulated			carrying
(In millions)	amount		amortization			amount
Definite-lived intangibles
Trademarks and trade names	$	66	$	(25	)	$	41
Intellectual property		721		(350	)		371
Customer and supplier relationships		759		(264	)		495
Total definite-lived intangibles		1,546		(639	)		907

Indefinite-lived intangibles
Trademarks and trade names		278		—			278
Total intangible assets	$	1,824	$	(639	)	$	1,185

Amortization expense recognized on intangible assets was $22 million and $23 million for the three months ended June 30, 2019 and 2018, respectively, and $65 million and $69 million for the nine months ended June 30, 2019 and 2018, respectively, and is included in the selling, general and administrative expense caption of the Statements of Consolidated Comprehensive Income (Loss). Estimated amortization expense for future periods is $86 million in 2019 (includes nine months actual and three months estimated), $86 million in 2020, $85 million in 2021, $85 million in 2022 and $85 million in 2023. The amortization expense for future periods is an estimate. Actual amounts may change from such estimated amounts due to fluctuations in foreign currency exchange rates, additional intangible asset acquisitions and divestitures, potential impairment, accelerated amortization, or other events.