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	Jul. 05, 2017	Apr. 10, 2017	Jan. 27, 2017	Jan. 19, 2017	Apr. 14, 2016	May 06, 2015	Jul. 31, 2017	Sep. 30, 2017	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2017	Apr. 03, 2019	Mar. 09, 2015	Jan. 23, 2015
Debt Instrument [Line Items]
Interest rate decrease (basis points)		(1.00%)
Debt obligations												$ 5,883.0	$ 5,743.1
Proceeds from issuance of private placement												275.0	462.8	$ 3,865.8
Repayment of debt												$ 135.0	315.7	2,408.8
Debt extinguishment costs													4.9
Secured debt ratio												4.50
Loss on extinguishment of debt											$ 4.9		4.9	18.2
Debt issuance cost												$ 114.8
Unamortized debt issuance costs associated with previous indebtedness													3.0	10.4
Unamortized debt issuance cost due to previous indebtedness												50.2	59.6
Accumulated amortization of debt issuance costs												64.6	54.5
Interest expense associated with the amortization of debt issuance costs												$ 10.1	$ 9.5	9.3
Maximum
Debt Instrument [Line Items]
Total indebtedness ratio												6.00
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Weighted average interest rate												4.20%	3.80%
Aggregate principal amount outstanding												$ 145.0
Debt obligations												145.0
Proceeds from revolving credit facility												275.0	$ 0.0
Outstanding letters of credit increased		$ 50.0
Remaining borrowing capacity												$ 296.5
Secured debt ratio												5.25
Revolving Credit Facility [Member] | Minimum
Debt Instrument [Line Items]
Percentage of excess cash flows committed to debt payments												35.00%
Percentage fee on unused capacity		0.25%
Revolving Credit Facility [Member] | Maximum
Debt Instrument [Line Items]
Percentage fee on unused capacity		0.375%
Letters of Credit [Member]
Debt Instrument [Line Items]
Percentage fee on unused capacity		0.25%
Outstanding letters of credit												$ 8.5
LIBOR [Member] | Letters of Credit [Member] | Minimum
Debt Instrument [Line Items]
Fee on outstanding letters of credit based upon leverage ratio		1.00%
LIBOR [Member] | Letters of Credit [Member] | Maximum
Debt Instrument [Line Items]
Fee on outstanding letters of credit based upon leverage ratio		1.75%
Tranche One [Member]
Debt Instrument [Line Items]
Debt obligations															$ 500.0
Tranche Two (B-2 Term Loan) [Member]
Debt Instrument [Line Items]
Proceeds from issuance of private placement				$ 310.7
Secured Debt [Member] | Revolving Credit Facility [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%
Secured Debt [Member] | Letters of Credit [Member] | Maximum
Debt Instrument [Line Items]
Percentage of excess cash flows committed to debt payments									50.00%
Term Loan Facility [Member]
Debt Instrument [Line Items]
Weighted average interest rate												4.60%	4.30%
Debt obligations							$ 2,500.0
Repriced percentage							99.75%
Repayment of debt					$ 570.1			$ 196.0		$ 310.7
Debt extinguishment costs														$ 18.2
Term Loan Facility [Member] | Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Aggregate principal amount outstanding												$ 1,758.0
Term Loan Facility [Member] | LIBOR [Member] | Minimum
Debt Instrument [Line Items]
Spread on interest rate		1.00%										1.00%
Term Loan Facility [Member] | LIBOR [Member] | Maximum
Debt Instrument [Line Items]
Spread on interest rate		1.75%
Incremental Term Loan [Member]
Debt Instrument [Line Items]
Debt obligations							$ 1,850.0
Repriced percentage							99.75%
Incremental Term Loan [Member] | Electric Lightwave
Debt Instrument [Line Items]
Interest rate decrease (basis points)			(0.25%)
Maturity period from incurrence							7 years
Debt obligations							$ 650.0
Incremental Term Loan [Member] | LIBOR [Member] | Electric Lightwave
Debt Instrument [Line Items]
Spread on interest rate			2.25%				2.50%
Incremental Term Loan [Member] | LIBOR [Member] | Electric Lightwave | Minimum
Debt Instrument [Line Items]
Spread on interest rate			1.00%				1.00%
Incremental Term Loan [Member] | Tranche One [Member]
Debt Instrument [Line Items]
Interest rate decrease (basis points)							(0.75%)
Maturity period from incurrence							4 years
Debt obligations	$ 150.0						$ 500.0
Incremental Term Loan [Member] | Tranche One [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Spread on interest rate	2.25%						2.00%
Incremental Term Loan [Member] | Tranche One [Member] | LIBOR [Member] | Minimum
Debt Instrument [Line Items]
Spread on interest rate	1.00%						0.00%
Incremental Term Loan [Member] | Tranche Two (B-2 Term Loan) [Member]
Debt Instrument [Line Items]
Interest rate decrease (basis points)							(0.25%)
Maturity period from incurrence							7 years
Debt obligations							$ 1,350.0
Incremental Term Loan [Member] | Tranche Two (B-2 Term Loan) [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Spread on interest rate							2.50%
Incremental Term Loan [Member] | Tranche Two (B-2 Term Loan) [Member] | LIBOR [Member] | Minimum
Debt Instrument [Line Items]
Spread on interest rate							1.00%
Unsecured Notes [Member] | 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%
Unsecured Notes [Member] | 6.00% Senior Notes due 2023 [Member]
Debt Instrument [Line Items]
Debt obligations												$ 1,430.0	$ 1,430.0
Interest rate of debt redeemed												6.00%
Unsecured Notes [Member] | 6.00% Senior Notes due 2023 [Member] | Zayo Group Ltd (ZGL) [Member]
Debt Instrument [Line Items]
Debt obligations																	$ 700.0
Unsecured Notes [Member] | 6.00% Senior Notes due 2023 [Member] | Zayo Capital [Member]
Debt Instrument [Line Items]
Debt obligations																$ 730.0
Unsecured Notes [Member] | 6.375% Senior Notes due 2025 [Member]
Debt Instrument [Line Items]
Debt obligations												$ 900.0	900.0
Interest rate of debt redeemed												6.375%
Unsecured Notes [Member] | 6.375% Senior Notes due 2025 [Member] | Zayo Group Ltd (ZGL) and Zayo Capital [Member]
Debt Instrument [Line Items]
Debt obligations								$ 550.0	$ 350.0
Repriced percentage								97.76%
Unsecured Notes [Member] | 5.75% Senior Notes due 2027 [Member]
Debt Instrument [Line Items]
Debt obligations				$ 300.0	$ 550.0							$ 1,650.0	$ 1,650.0
Debt price percentage				104.25%		104.00%
Interest rate of debt redeemed				5.75%	5.75%							5.75%
Unsecured Notes [Member] | 5.75% Senior Notes due 2027 [Member] | Zayo Group Ltd (ZGL) and Zayo Capital [Member]
Debt Instrument [Line Items]
Debt obligations						$ 800.0
Interest rate of debt redeemed						5.75%