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. 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	Jan. 27, 2017	Mar. 09, 2015	Jan. 23, 2015
Debt Instrument [Line Items]
Interest rate decrease (basis point)		(1.00%)
Debt obligations											$ 5,883.0	$ 5,743.1
Outstanding letters of credit											8.5
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]
Percentage fee on unused capacity		0.25%
Weighted average interest rate											4.20%	3.80%
Aggregate principal amount outstanding											$ 145.0
Debt obligations											145.0
Outstanding letters of credit increased		$ 50.0
Remaining borrowing capacity											296.5
Proceeds from revolving credit facility											$ 275.0	$ 0.0
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%
Secured debt ratio											5.25
Letters of Credit [Member]
Debt Instrument [Line Items]
Outstanding letters of credit											$ 8.5
Letters of Credit [Member] \| Minimum
Debt Instrument [Line Items]
Fee on outstanding letters of credit based upon leverage ratio		1.00%
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								25.00%
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%
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
Proceeds from issuance of private placement	$ 150.0
Repriced percentage						99.75%
Incremental Term Loan [Member] \| Electric Lightwave [Member]
Debt Instrument [Line Items]
Interest rate decrease (basis point)			(0.25%)
Debt obligations						$ 650.0
Incremental Term Loan [Member] \| LIBOR [Member] \| Electric Lightwave [Member]
Debt Instrument [Line Items]
Spread on interest rate			2.25%			2.50%
Incremental Term Loan [Member] \| LIBOR [Member] \| Minimum \| Electric Lightwave [Member]
Debt Instrument [Line Items]
Spread on interest rate			1.00%			1.00%
Incremental Term Loan [Member] \| Tranche One [Member]
Debt Instrument [Line Items]
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]
Interest rate decrease (basis point)						(0.25%)
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 point)						(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 Debt [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 Debt [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 Debt [Member] \| 6.00% Senior Notes due 2023 [Member] \| Zayo Group Ltd (ZGL) [Member]
Debt Instrument [Line Items]
Debt obligations																	$ 700.0
Unsecured Debt [Member] \| 6.00% Senior Notes due 2023 [Member] \| Zayo Capital [Member]
Debt Instrument [Line Items]
Debt obligations																$ 730.0
Unsecured Debt [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 Debt [Member] \| 6.375% Senior Notes due 2025 [Member] \| Zayo Group Ltd And Zayo Capital [Member]
Debt Instrument [Line Items]
Debt obligations							$ 550.0	$ 350.0
Repriced percentage							97.76%
Unsecured Debt [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 Debt [Member] \| 5.75% Senior Notes due 2027 [Member] \| Zayo Group Ltd And Zayo Capital [Member]
Debt Instrument [Line Items]
Debt obligations															$ 800.0
Interest rate of debt redeemed															5.75%

ZAYO GROUP LLC 10-K