(6) INTANGIBLE ASSETS

Identifiable intangible assets as of June 30, 2017 and 2016 were as follows:


	Gross Carrying Amount		Accumulated Amortization		Net
	(in millions)
June 30, 2017
Finite-Lived Intangible Assets
Customer relationships	$	1,477.7	$	(308.6)	$	1,169.1
Underlying rights		1.6		(0.4)		1.2
Total		1,479.3		(309.0)		1,170.3
Indefinite-Lived Intangible Assets
Certifications		3.5		—		3.5
Underlying Rights		14.8		—		14.8
Total	$	1,497.6	$	(309.0)	$	1,188.6
June 30, 2016
Finite-Lived Intangible Assets
Customer relationships	$	1,143.6	$	(228.8)	$	914.8
Trade names		0.2		(0.2)		—
Underlying rights		1.6		(0.3)		1.3
Total		1,145.4		(229.3)		916.1
Indefinite-Lived Intangible Assets
Certifications		3.5		—		3.5
Underlying Rights		15.3		—		15.3
Total	$	1,164.2	$	(229.3)	$	934.9

The weighted average remaining amortization period for the Company’s customer relationships asset is 15.4 years. The Company has determined that certain underlying rights (including easements) and the certifications have indefinite lives. The amortization period for underlying rights (including easements) is 20 years. The amortization of intangible assets for the years ended June 30, 2017, 2016 and 2015 was $80.0 million, $75.8 million, and $54.8 million, respectively.

During the years ended June 30, 2017 and 2016, the Company wrote off $0.3 million and $1.8 million in fully amortized intangible assets, respectively. Estimated future amortization of finite-lived intangible assets is as follows:


June 30,
	(in millions)
2018	$	89.5
2019		88.2
2020		83.2
2021		81.1
2022		79.3
Thereafter		749.0
Total	$	1,170.3