(6) INTANGIBLE ASSETS

Identifiable intangible assets as of June 30, 2018 and 2017 were as follows:


	Gross Carrying Amount		Accumulated Amortization		Net
	(in millions)
June 30, 2018
Finite-Lived Intangible Assets
Customer relationships	$	1,597.0	$	(405.6)	$	1,191.4
Underlying rights and other		2.7		(0.6)		2.1
Total		1,599.7		(406.2)		1,193.5
Indefinite-Lived Intangible Assets
Certifications		3.5		—		3.5
Underlying rights and other		15.1		—		15.1
Total	$	1,618.3	$	(406.2)	$	1,212.1
June 30, 2017
Finite-Lived Intangible Assets
Customer relationships	$	1,477.7	$	(308.6)	$	1,169.1
Underlying rights and other		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 and other		14.8		—		14.8
Total	$	1,497.6	$	(309.0)	$	1,188.6

The weighted average remaining amortization period for the Company’s customer relationships asset is 15.1 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, 2018, 2017 and 2016 was $97.2 million, $80.0 million, and $75.8 million, respectively.

During the years ended June 30, 2018 and 2017, the Company wrote off nil and $0.3 million in fully amortized intangible assets, respectively. Estimated future amortization of finite-lived intangible assets is as follows:



June 30,	(in millions)
2019	$	94.6
2020		89.5
2021		87.3
2022		85.3
2023		83.6
Thereafter		753.2
Total	$	1,193.5

In connection with the Company’s acquisition of the McLean Data Center, the Company assumed an operating lease at unfavorable terms when compared to prevailing market rates. As a result, the Company recorded an unfavorable lease obligation of $9.8 million, in other long-term liabilities, which will be amortized through the term of the lease which ends in March 2024. Amortization of the liability was $0.5 million for the year ended June 20, 2018. Estimated future amortization of the lease liability is expected to be $1.6 million for the years ended June 30, 2019, 2020, 2021, 2022, and 2023, and $1.3 million for the year ended June 30, 2024.