Note 7. Intangible Assets, net

Intangible assets, net, consisted of the following:


		December 31, 2018				December 31, 2017
	Weighted Average	Gross				Gross
	Amortizable	Carrying		Accumulated		Carrying		Accumulated
	Life in Years	Amount		Amortization		Amount		Amortization
		(in millions)
Intangible assets subject to amortization:
Covenants not to compete	4.8	$	0.8	$	(0.4)	$	0.8	$	(0.4)
Customer lists/relationships	14.9		707.3		(393.4)		745.0		(391.3)
Software	10.0		8.1		(8.1)		8.1		(8.1)
Other	7.4		1.0		(0.9)		6.3		(5.9)
			717.2		(402.8)		760.2		(405.7)
Intangible assets not subject to amortization:
Trade names			757.6		—		757.6		—
		$	1,474.8	$	(402.8)	$	1,517.8	$	(405.7)

Intangible assets recorded in connection with our 2018 acquisitions were $43.3 million (see Note 2 — “Acquisitions”). A total of $18.3 million was allocated to the trade names acquired, which is not subject to amortization.

During 2018, we recognized impairment losses of $16.5 million and $16.7 million on our trade name and customer relationship intangible assets, respectively, related to one of our energy businesses. Impairment losses of $36.4 million related to eight of our trade name intangible assets were recognized in 2016. See Note 18 — “Impairment and Restructuring Charges” for further discussion of our impairment losses.

Amortization expense for intangible assets amounted to $45.8 million, $50.6 million and $54.1 million in 2018, 2017 and 2016, respectively. Foreign currency translation losses related to intangible assets, net in 2018 were $4.1 million.

The following is a summary of estimated aggregate amortization expense for each of the next five years:


	(in millions)
2019	$	43.0
2020		43.0
2021		41.3
2022		36.7
2023		30.7