6. Intangible Assets, net

Intangible assets, net consisted of the following:


		June 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	4.8	$	0.8	$	(0.5)	$	0.8	$	(0.4)
Customer lists/relationships	14.5		751.1		(413.3)		745.0		(391.3)
Software	10.0		8.1		(8.1)		8.1		(8.1)
Other	4.7		5.9		(5.7)		6.3		(5.9)
			765.9		(427.6)		760.2		(405.7)
Intangible assets not subject to amortization:
Trade names			763.2		—		757.6		—
		$	1,529.1	$	(427.6)	$	1,517.8	$	(405.7)

Intangible assets recorded in connection with our acquisition of DuBose were $15.7 million as of June 30, 2018 (see Note 3—“Acquisitions”). A total of $6.6 million was allocated to the trade names acquired, which is not subject to amortization.

We recognized amortization expense for intangible assets of $23.7 million and $26.7 million for the six months ended June 30, 2018 and 2017, respectively. Foreign currency translation losses related to intangible assets, net, were $2.3 million during the six months ended June 30, 2018.

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


	(in millions)
2018 (remaining six months)	$	23.7
2019		47.3
2020		47.3
2021		43.2
2022		35.1
2023		29.1