6. Intangible Assets, net

Intangible assets, net consisted of the following:


		March 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	14.5		752.6		(402.4)		745.0		(391.3)
Software	10.0		8.1		(8.1)		8.1		(8.1)
Other	5.7		7.6		(6.6)		6.3		(5.9)
			769.1		(417.5)		760.2		(405.7)
Intangible assets not subject to amortization:
Trade names			763.9		—		757.6		—
		$	1,533.0	$	(417.5)	$	1,517.8	$	(405.7)

Intangible assets recorded in connection with our acquisition of DuBose were $15.7 million as of March 31, 2018 (see Note 3—“Acquisitions”). A total of $6.6 million was allocated to the trade names acquired, which are not subject to amortization.

We recognized amortization expense for intangible assets of $11.8 million and $13.4 million for the first quarters of 2018 and 2017, respectively. Foreign currency translation losses related to intangible assets, net, were $0.9 million for the first quarter of 2018.

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


	(in millions)
2018 (remaining nine months)	$	35.7
2019		47.5
2020		47.5
2021		43.4
2022		35.2
2023		29.1