GOODWILL AND INTANGIBLE ASSETS

The aggregate carrying amount of goodwill was $148.5 million as of June 30, 2018, and $148.3 million as of December 31, 2017.

Balances of intangible assets, net, excluding goodwill as of June 30, 2018, are as follows:



	December 31, 2017		Additions		Amortization	June 30, 2018
Millions
Intangible Assets
Definite-Lived Intangible Assets
Customer Relationships		$54.7		$0.2	$(2.1)		$52.8
Developed Technology and Other (a)	6.3		2.2		(0.6)	7.9
Total Definite-Lived Intangible Assets	61.0		2.4		(2.7)	60.7
Indefinite-Lived Intangible Assets
Trademarks and Trade Names	16.6		—		n/a	16.6
Total Intangible Assets		$77.6		$2.4	$(2.7)		$77.3


(a)	Developed Technology and Other includes patents, non-compete agreements, land easements and trade names with finite lives.

Customer relationships have a remaining useful life of approximately 20 years, and developed technology and other have remaining useful lives ranging from approximately 1 year to approximately 11 years (weighted average of approximately 6 years). The weighted average remaining useful life of all definite-lived intangible assets as of June 30, 2018, is approximately 18 years.

NOTE 4. GOODWILL AND INTANGIBLE ASSETS (Continued)

Amortization expense for intangible assets was $1.3 million and $2.7 million for the quarter and six months ended June 30, 2018, respectively ($1.4 million and $2.8 million for the quarter and six months ended June 30, 2017, respectively). Accumulated amortization was $17.5 million as of June 30, 2018 ($14.8 million as of December 31, 2017). The estimated amortization expense for definite-lived intangible assets for the remainder of 2018 is $2.8 million. Estimated annual amortization expense for definite‑lived intangible assets is $5.2 million in 2019, $5.0 million in 2020, $4.9 million in 2021, $4.6 million in 2022 and $38.2 million thereafter.