December 31, 2020	Gross Cost		Accumulated Amortization			Net Cost
Technologies & licenses		934		(623	)		311
Purchased & internally developed software		537		(461	)		76
Technologies in progress		56		—			56
Other intangible assets		72		(70	)		2
Total		1,599		(1,154	)		445

December 31, 2019	Gross Cost		Accumulated Amortization			Net Cost
Technologies & licenses		699		(578	)		121
Purchased & internally developed software		486		(427	)		59
Technologies in progress		119		—			119
Other intangible assets		70		(70	)		—
Total		1,374		(1,075	)		299

As described in Note 7, the acquisitions completed in 2020 resulted in the recognition of technologies of $108 million, reported on the line “Technologies & licenses”, $10 million of technology in progress reported on the line “Technologies in progress” and $2 million of other intangible assets reported on the line “Other intangible assets”.

Furthermore, the 2019 acquisition of Norstel resulted in the recognition of technology in progress for $86 million reported on the line “Technologies in progress” as at December 31, 2019. The acquired technology was reported on the line “Technologies & licenses” as at December 31, 2020.

The line “Technologies in progress” in the table above also includes internally developed software under construction and software not ready for use.

The amortization expense in 2020, 2019 and 2018 was $79 million, $69 million and $64 million, respectively.

The estimated amortization expense of the existing intangible assets for the following years is:

Year
2021			97
2022			82
2023			64
2024			49
2025			35
Thereafter			118
Total			445

In 2020, 2019 and 2018, the Company impaired $4 million, $4 million and $2 million, respectively, of acquired licenses and technologies for which it was determined that they had no alternative future use.