LONG-TERM DEBT (Narrative) (Details) - USD ($) $ in Millions												1 Months Ended	3 Months Ended			12 Months Ended
	Feb. 26, 2018	Dec. 22, 2017	Jul. 20, 2017	Apr. 10, 2017	Jan. 27, 2017	Jan. 19, 2017	Jul. 22, 2016	Jul. 05, 2016	Apr. 14, 2016	Jan. 15, 2016	May 06, 2015	Jul. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2016	Mar. 09, 2015	Jan. 23, 2015
Debt Instrument [Line Items]
Interest rate decrease (basis point)		(1.00%)
Payments of Debt Extinguishment Costs																		$ 20.3
Debt obligations														$ 5,608.7		$ 5,743.1	$ 5,608.7
Proceeds from issuance of private placement								$ 310.7								462.8	3,865.8	929.3
Repayment of debt																315.7	2,408.8	535.0
Loss on extinguishment of debt													$ 4.9	13.7	$ 4.5	4.9	18.2	33.8
Unamortized Debt Issuance Costs Associated with Previous Indebtedness																3.0	10.4	11.4
Unamortized debt issuance cost due to previous indebtedness														67.8		59.6	67.8
Debt issuance costs																114.1
Accumulated amortization														$ 45.1		54.5	45.1
Interest expense associated with the amortization of debt issuance costs																$ 9.5	$ 9.3	$ 10.0
Revolver [Member]
Debt Instrument [Line Items]
Percentage of excess cash flows committed to debt payments																35.00%
Outstanding letters of credit																$ 0.0
Outstanding letters of credit increased		$ 50.0
Remaining borrowing capacity																$ 441.9
Secured debt ratio																5.25
Letters of Credit [Member]
Debt Instrument [Line Items]
Unused commitment, percentage		0.25%
Outstanding letters of credit																$ 8.1
Secured Debt [Member] | Revolver [Member]
Debt Instrument [Line Items]
Line of credit facility maximum borrowing capacity											$ 450.0
Secured Debt [Member] | Letters of Credit [Member]
Debt Instrument [Line Items]
Quarterly principal payments per quarter, in basis points											0.25%
Percentage of excess cash flows committed to debt payments											50.00%
Term Loan Facility
Debt Instrument [Line Items]
Interest rate decrease (basis point)							(0.75%)
Weighted average interest rate														3.40%		4.30%	3.40%
Debt obligations						$ 2,500.0
Repriced percentage						99.75%
Repayment of debt				$ 570.1					$ 196.0			$ 310.7
Term Loan Facility | Revolver [Member]
Debt Instrument [Line Items]
Aggregate principal amount outstanding																$ 1,763.0
Incremental Term Loan [Member]
Debt Instrument [Line Items]
Line of credit facility maximum borrowing capacity	$ 150.0									$ 400.0
Priced percent of additional line of credit										99.00%
Debt obligations						$ 1,850.0
Repriced percentage						99.75%
Incremental Term Loan [Member] | Electric Lightwave
Debt Instrument [Line Items]
Interest rate decrease (basis point)			(0.25%)
Maturity period from incurrence						7 years
Debt obligations						$ 650.0
2023 Unsecured Notes [Member]
Debt Instrument [Line Items]
Secured debt ratio																4.50
Total indebtedness ratio																6.00
Minimum | Revolver [Member]
Debt Instrument [Line Items]
Unused commitment, percentage		0.25%
Minimum | Term Loan Facility | Revolver [Member]
Debt Instrument [Line Items]
Effective Interest rates (in percent)														3.80%		3.80%	3.80%
Maximum | Revolver [Member]
Debt Instrument [Line Items]
Unused commitment, percentage		0.375%
Tranche One [Member] | Incremental Term Loan [Member]
Debt Instrument [Line Items]
Interest rate decrease (basis point)						(0.75%)
Maturity period from incurrence						4 years
Debt obligations	$ 150.0					$ 500.0
Tranche Two [Member] | Incremental Term Loan [Member]
Debt Instrument [Line Items]
Interest rate decrease (basis point)						(0.25%)
Maturity period from incurrence						7 years
Debt obligations						$ 1,350.0
6.00% Senior Notes due 2023 [Member]
Debt Instrument [Line Items]
Debt obligations														$ 1,430.0		$ 1,430.0	$ 1,430.0
Interest rate of debt redeemed														6.00%		6.00%	6.00%
6.00% Senior Notes due 2023 [Member] | Unsecured Notes
Debt Instrument [Line Items]
Debt instrument, redemption price, percentage																100.00%
6.00% Senior Notes due 2023 [Member] | Unsecured Notes | Zayo Group Ltd (ZGL) [Member]
Debt Instrument [Line Items]
Debt obligations																				$ 700.0
6.00% Senior Notes due 2023 [Member] | Unsecured Notes | Zayo Capital [Member]
Debt Instrument [Line Items]
Debt obligations																			$ 730.0
6.375% New Senior Notes due 2025 [Member] | Unsecured Notes | Zayo Group Ltd (ZGL) and Zayo Capital [Member]
Debt Instrument [Line Items]
Debt obligations									550.0
6.375% Senior Notes due 2025 [Member]
Debt Instrument [Line Items]
Debt obligations														$ 900.0		$ 900.0	$ 900.0
Interest rate of debt redeemed														6.375%		6.375%	6.375%
6.375% Senior Notes due 2025 [Member] | Unsecured Notes | Zayo Group Ltd (ZGL) and Zayo Capital [Member]
Debt Instrument [Line Items]
Debt obligations									$ 350.0
Repriced percentage									97.76%
5.75% Senior Unsecured Notes due 2027 [Member]
Debt Instrument [Line Items]
Debt obligations				$ 550.0	$ 800.0			$ 300.0						$ 1,350.0		$ 1,650.0	$ 1,350.0
Debt price percentage					104.00%			104.25%
Interest rate of debt redeemed														5.75%		5.75%	5.75%
10.125% Senior Notes due 2020 [Member]
Debt Instrument [Line Items]
Repayment of debt									$ 325.6
Redemption premium and accrued interest included in redemption amount									$ 20.3
Interest rate of debt redeemed									10.125%
LIBOR | Term Loan Facility
Debt Instrument [Line Items]
Spread on interest rate							2.75%
LIBOR | Incremental Term Loan [Member]
Debt Instrument [Line Items]
Spread on interest rate										3.50%
LIBOR | Incremental Term Loan [Member] | Electric Lightwave
Debt Instrument [Line Items]
Spread on interest rate			2.25%			2.50%
LIBOR | Minimum | Letters of Credit [Member]
Debt Instrument [Line Items]
Fee on outstanding letters of credit based upon leverage ratio		1.00%
LIBOR | Minimum | Term Loan Facility
Debt Instrument [Line Items]
Spread on interest rate		1.00%					1.00%									1.00%
LIBOR | Minimum | Incremental Term Loan [Member]
Debt Instrument [Line Items]
Spread on interest rate										1.00%
LIBOR | Minimum | Incremental Term Loan [Member] | Electric Lightwave
Debt Instrument [Line Items]
Spread on interest rate			1.00%			1.00%
LIBOR | Maximum | Letters of Credit [Member]
Debt Instrument [Line Items]
Fee on outstanding letters of credit based upon leverage ratio		1.75%
LIBOR | Maximum | Term Loan Facility
Debt Instrument [Line Items]
Spread on interest rate		1.75%
LIBOR | Tranche One [Member] | Incremental Term Loan [Member]
Debt Instrument [Line Items]
Spread on interest rate	2.25%					2.00%
LIBOR | Tranche One [Member] | Minimum | Incremental Term Loan [Member]
Debt Instrument [Line Items]
Spread on interest rate	1.00%					0.00%
LIBOR | Tranche Two [Member] | Incremental Term Loan [Member]
Debt Instrument [Line Items]
Spread on interest rate						2.50%
LIBOR | Tranche Two [Member] | Minimum | Incremental Term Loan [Member]
Debt Instrument [Line Items]
Spread on interest rate						1.00%