LONG-TERM DEBT (Narrative) (Details) - USD ($) $ in Millions										1 Months Ended	3 Months Ended		12 Months Ended
	Dec. 22, 2017	Jul. 20, 2017	Jul. 05, 2017	Apr. 10, 2017	Jan. 19, 2017	Jul. 22, 2016	Apr. 14, 2016	Jan. 15, 2016	May 06, 2015	Jul. 31, 2017	Sep. 30, 2018	Sep. 30, 2017	Jun. 30, 2018	Jan. 27, 2017	Mar. 09, 2015	Jan. 23, 2015
Debt Instrument [Line Items]
Payments of Debt Extinguishment Costs												$ 4.9
Debt obligations											$ 5,741.8		$ 5,743.1
Proceeds from issuance of private placement			$ 310.7									312.8
Repayment of debt											$ 1.3	311.9
Loss on extinguishment of debt												$ 4.9
Secured debt ratio											4.50
Debt issuance costs											$ 114.1
Accumulated amortization											57.0		54.5
Interest expense associated with the amortization of debt issuance costs											$ 2.5		$ 2.3
Revolver [Member]
Debt Instrument [Line Items]
Weighted average interest rate											4.00%		3.80%
Secured Debt [Member] | Revolver [Member]
Debt Instrument [Line Items]
Line of credit facility maximum borrowing capacity									$ 450.0
Remaining borrowing capacity											$ 441.9
Percentage of excess revolver committed to debt payments											35.00%
Secured debt ratio											5.25
Secured Debt [Member] | Letters of Credit [Member]
Debt Instrument [Line Items]
Outstanding letters of credit											$ 8.1
Term Loan Facility
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%
Weighted average interest rate											4.40%		4.30%
Aggregate principal amount outstanding											$ 1,761.8
Debt obligations					$ 2,500.0
Repriced percentage					99.75%
Repayment of debt							$ 196.0
Incremental Term Loan [Member]
Debt Instrument [Line Items]
Line of credit facility increase to borrowing capacity								$ 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%)
Debt obligations					$ 650.0
Maximum
Debt Instrument [Line Items]
Total indebtedness ratio											6.00
Tranche One [Member] | Incremental Term Loan [Member]
Debt Instrument [Line Items]
Interest rate decrease (basis point)					(0.75%)
Debt obligations					$ 500.0
Tranche Two (B-2 Term Loan) [Member]
Debt Instrument [Line Items]
Repayment of debt										$ 310.7
Tranche Two (B-2 Term Loan) [Member] | Incremental Term Loan [Member]
Debt Instrument [Line Items]
Interest rate decrease (basis point)					(0.25%)
Debt obligations					$ 1,350.0
Repayment of debt				$ 570.1
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%
Unamortized debt issuance cost due to previous indebtedness											$ 57.1		59.6
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% Senior Notes due 2025 [Member]
Debt Instrument [Line Items]
Debt obligations											$ 900.0		900.0
Interest rate of debt redeemed											6.375%
Incremental 2025 Notes [Member]
Debt Instrument [Line Items]
Debt price percentage							97.76%
Incremental 2025 Notes [Member] | Unsecured Notes | Zayo Group Ltd (ZGL) and Zayo Capital [Member]
Debt Instrument [Line Items]
Debt obligations							$ 550.0		$ 350.0
5.75% Senior Unsecured Notes due 2027 [Member]
Debt Instrument [Line Items]
Debt obligations			$ 300.0	$ 550.0							$ 1,650.0		$ 1,650.0	$ 800.0
Debt price percentage			104.25%	104.00%
Interest rate of debt redeemed											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%
Repricing Amendment No. 3 [member] | Zayo Group Ltd (ZGL) and Zayo Capital [Member]
Debt Instrument [Line Items]
Line of credit facility maximum borrowing capacity	$ 50.0
Interest rate decrease (basis point)	(1.00%)
Percentage fee on undrawn amount	0.25%
Fee on outstanding letters of credit based upon leverage ratio	1.75%
Repricing Amendment No. 3 [member] | Minimum | Zayo Group Ltd (ZGL) and Zayo Capital [Member]
Debt Instrument [Line Items]
Percentage fee on unused capacity	0.25%
Percentage fee on outstanding amounts	1.00%
Repricing Amendment No. 3 [member] | Maximum | Zayo Group Ltd (ZGL) and Zayo Capital [Member]
Debt Instrument [Line Items]
Percentage fee on unused capacity	0.375%
Percentage fee on outstanding amounts	1.75%
LIBOR | Incremental Term Loan [Member]
Debt Instrument [Line Items]
Interest rate decrease (basis point)						(0.75%)
Spread on interest rate						2.75%		3.50%
LIBOR | Incremental Term Loan [Member] | Electric Lightwave
Debt Instrument [Line Items]
Spread on interest rate		2.25%			2.50%
LIBOR | Minimum | Term Loan Facility
Debt Instrument [Line Items]
Spread on interest rate											1.00%
LIBOR | Minimum | Incremental Term Loan [Member]
Debt Instrument [Line Items]
Spread on interest rate						1.00%		1.00%
LIBOR | Minimum | Incremental Term Loan [Member] | Electric Lightwave
Debt Instrument [Line Items]
Spread on interest rate		1.00%			1.00%
LIBOR | Tranche One [Member] | Incremental Term Loan [Member]
Debt Instrument [Line Items]
Spread on interest rate					2.00%
LIBOR | Tranche One [Member] | Minimum | Incremental Term Loan [Member]
Debt Instrument [Line Items]
Spread on interest rate					0.00%
LIBOR | Tranche Two (B-2 Term Loan) [Member] | Incremental Term Loan [Member]
Debt Instrument [Line Items]
Spread on interest rate					2.50%
LIBOR | Tranche Two (B-2 Term Loan) [Member] | Minimum | Incremental Term Loan [Member]
Debt Instrument [Line Items]
Spread on interest rate					1.00%
LIBOR | Repricing Amendment No. 3 [member] | Zayo Group Ltd (ZGL) and Zayo Capital [Member]
Debt Instrument [Line Items]
Spread on interest rate	1.00%