6. Intangible Assets, net

Intangible assets, net consisted of the following:


		September 30, 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	4.7	$	0.9	$	(0.5)	$	0.8	$	(0.4)
Customer lists/relationships	15.1		698.2		(384.6)		745.0		(391.3)
Software	10.0		8.1		(8.1)		8.1		(8.1)
Other	7.6		1.0		(0.9)		6.3		(5.9)
			708.2		(394.1)		760.2		(405.7)
Intangible assets not subject to amortization:
Trade names			750.4		—		757.6		—
		$	1,458.6	$	(394.1)	$	1,517.8	$	(405.7)

Intangible assets recorded in connection with our acquisitions of KMS and DuBose were $22.9 million as of September 30, 2018 (see Note 3—“Acquisitions”). A total of $10.0 million was allocated to the trade names acquired, which is not subject to amortization.

We recognized amortization expense for intangible assets of $35.1 million and $38.9 million for the nine months ended September 30, 2018 and 2017, respectively. Foreign currency translation losses related to intangible assets, net, were $1.5 million during the nine months ended September 30, 2018.

During the three-month and nine-month periods ended September 30, 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. See Note 12—“Impairment and Restructuring Charges” for further discussion of our impairment losses.

The following is a summary of estimated aggregate amortization expense for the remaining three months of 2018 and each of the succeeding five years:


	(in millions)
2018 (remaining three months)	$	10.5
2019		41.9
2020		41.9
2021		40.2
2022		35.5
2023		29.5