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. 19, 2017	Jul. 05, 2017	Apr. 10, 2017	Jan. 19, 2017	Apr. 14, 2016	May 06, 2015	Jan. 23, 2015	Jul. 31, 2017	Sep. 30, 2019	Sep. 30, 2018	Jun. 30, 2019	Apr. 03, 2019	Jan. 27, 2017	Mar. 09, 2015
Debt Instrument [Line Items]
Debt obligations												$ 5,861.8		$ 5,883.0
Repayment of debt												$ 21.3	$ 1.3
Secured debt ratio												4.50
Debt issuance cost												$ 114.8
Debt Issuance Costs, Net												47.6		50.2
Accumulated amortization of debt issuance costs												67.3		64.6
Interest expense associated with the amortization of debt issuance costs												$ 2.7		$ 2.5
Maximum
Debt Instrument [Line Items]
Total indebtedness ratio												6.00
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Weighted average interest rate												3.80%		4.20%
Debt obligations												$ 125.0		$ 145.0
Proceeds from revolving credit facility												0.0	$ 0.0
Outstanding letters of credit increased		$ 50.0
Remaining borrowing capacity												$ 316.4
Secured debt ratio												5.25
Revolving Credit Facility [Member] | Minimum
Debt Instrument [Line Items]
Percentage of excess revolver committed to debt payments												35.00%
Letters of Credit [Member]
Debt Instrument [Line Items]
Percentage fee on undrawn amount		0.25%
Outstanding letters of credit												$ 8.6
Letters of Credit [Member] | Minimum
Debt Instrument [Line Items]
Percentage fee on unused capacity		0.25%
Percentage fee on outstanding amounts		1.00%
Letters of Credit [Member] | Maximum
Debt Instrument [Line Items]
Percentage fee on unused capacity		0.375%
Percentage fee on outstanding amounts		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
6.375% Senior Notes due 2025 [Member]
Debt Instrument [Line Items]
Interest rate of debt redeemed								6.375%
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]
Interest rate decrease (basis points)		(1.00%)
Weighted average interest rate												4.20%		4.60%
Aggregate principal amount outstanding												$ 125.0
Debt obligations							$ 2,500.0
Repriced percentage							99.75%
Repayment of debt						$ 570.1		$ 196.0			$ 310.7
Term Loan Facility [Member] | Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Aggregate principal amount outstanding												$ 1,756.8
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]
Fee on outstanding letters of credit based upon leverage ratio		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]
Interest rate decrease (basis points)										6.00%
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
Debt price 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%