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, 2019 assessment, Ashland determined that its reporting units for the allocation of goodwill were its two reportable segments: Specialty Ingredients and Intermediates and Solvents. At that time, Ashland determined no additional impairment existed.

The following is a progression of goodwill by reportable segment for the three months ended December 31, 2019.

	Specialty		Intermediates
(In millions)	Ingredients		and Solvents		(a)	Total
Balance at September 30, 2019	$	2,253	$	—		$	2,253
Currency translation		21		—			21
Balance at December 31, 2019	$	2,274	$	—		$	2,274

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 December 31, 2019 and September 30, 2019.

	December 31, 2019
	Gross					Net
	carrying		Accumulated			carrying
(In millions)	amount		amortization			amount
Definite-lived intangibles
Trademarks and trade names	$	66	$	(30	)	$	36
Intellectual property		717		(405	)		312
Customer and supplier relationships		750		(304	)		446
Total definite-lived intangibles		1,533		(739	)		794

Indefinite-lived intangibles
Trademarks and trade names		278		—			278
Total intangible assets	$	1,811	$	(739	)	$	1,072

	September 30, 2019
	Gross					Net
	carrying		Accumulated			carrying
(In millions)	amount		amortization			amount
Definite-lived intangibles
Trademarks and trade names	$	66	$	(29	)	$	37
Intellectual property		712		(391	)		321
Customer and supplier relationships		744		(292	)		452
Total definite-lived intangibles		1,522		(712	)		810

Indefinite-lived intangibles
Trademarks and trade names		278		—			278
Total intangible assets	$	1,800	$	(712	)	$	1,088

Amortization expense recognized on intangible assets was $21 million and $22 million for the three months ended December 31, 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 $85 million in 2020 (includes three months actual and nine months estimated), $85 million in 2021, $84 million in 2022, $84 million in 2023 and $70 million in 2024. 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.