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Name:	zayo_FinancingAmendmentAxis=zayo_SixthAmendmentMember
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Long-Term Debt (Narrative) (Detail) (USD $) In Millions, unless otherwise specified	0 Months Ended	3 Months Ended		6 Months Ended	9 Months Ended		12 Months Ended	0 Months Ended		3 Months Ended	0 Months Ended
	May 16, 2014	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014	Mar. 31, 2015	Mar. 31, 2014	Jun. 30, 2014	Nov. 26, 2013	Jul. 02, 2012	Dec. 31, 2014	Jan. 23, 2015	Aug. 13, 2012	Dec. 15, 2014	Mar. 09, 2015
Debt Instrument [Line Items]
Debt obligations		$ 3,751.1us-gaap_DebtInstrumentCarryingAmount			$ 3,751.1us-gaap_DebtInstrumentCarryingAmount		$ 3,260.8us-gaap_DebtInstrumentCarryingAmount	$ 1,749.8us-gaap_DebtInstrumentCarryingAmount
Debt issuance cost	3.2us-gaap_DebtIssuanceCosts				102.1us-gaap_DebtIssuanceCosts
Loss on extinguishment of debt		(54.9)			(85.8)	(1.9)
Debt instrument carrying amount	2,015.9us-gaap_LongTermDebt	3,740.6us-gaap_LongTermDebt			3,740.6us-gaap_LongTermDebt		3,240.2us-gaap_LongTermDebt
Debt instrument, call feature	99.5%
Redemption premium					23.8us-gaap_RedemptionPremium
Proceeds from debt					1,437.3	150.0
Accretion of premium on debt				28.2			25.4
Unamortized debt issuance cost		73.9us-gaap_UnamortizedDebtIssuanceExpense			73.9us-gaap_UnamortizedDebtIssuanceExpense		89.4us-gaap_UnamortizedDebtIssuanceExpense
Unamortized debt issuance related interest		3.6	3.5		11.1	10.3
Interest Rate Swap [Member]
Debt Instrument [Line Items]
Notional amount of derivative												750.0
Derivative, maturity date					Jun. 30, 2017
Derivative, fixed interest rate												1.67%
Derivative, floor rate												1.25%
Change in fair value of interest rate swap		2.3			1.8	1.9
Interest Rate Derivative Assets, at Fair Value		3.8			3.8		2.0
payment on interest rate derivatives		0.8	0.8		2.4	1.6
Senior Secured And Senior Unsecured Notes
Debt Instrument [Line Items]
Loss on extinguishment of debt		(16.1)			(23.2)
Fifth Amendment [Member]
Debt Instrument [Line Items]
Proceeds from Issuance of Long-term Debt								150.0
Sixth Amendment [Member]
Debt Instrument [Line Items]
Proceeds from Issuance of Long-term Debt	275.0
Revolver [Member]
Debt Instrument [Line Items]
Debt instrument, face amount									250.0
Revolver [Member] \| Fifth Amendment [Member] \| LIBOR [Member]
Debt Instrument [Line Items]
Percentage over variable rate								2.75%
Term Loan Facility [Member]
Debt Instrument [Line Items]
Discount on debt									30.0
Payment towards principal					5.1
Percentage of excess cash flows committed to debt payments					50.00%
Debt instrument, maturity date					Jul. 31, 2019
Percentage over variable rate		4.00%			4.00%		4.00%
Term Loan Facility [Member] \| LIBOR [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Revolver interest rate								3.00%
Term Loan Facility [Member] \| LIBOR [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Percentage over variable rate								1.00%
Senior Secured Notes [Member]
Debt Instrument [Line Items]
Debt instrument, face amount									750.0
Debt instrument, interest rate									8.125%				8.125%
Debt Instrument Maturity Year									2020
Redemption of notes													75.0
Redemption price as percentage													108.00%
Senior Unsecured Notes [Member]
Debt Instrument [Line Items]
Debt instrument, face amount									500.0
Debt instrument, interest rate									10.125%				10.125%
Debt Instrument Maturity Year									2020
Redemption of notes													174.4
Redemption price as percentage													110.00%
Senior Unsecured Notes [Member] \| Debt redemption in part before April 1, 2018 [Member]
Debt Instrument [Line Items]
Debt instrument, redemption price, percentage					103.19%
Term Loan Facility [Member] \| Fourth Amendment
Debt Instrument [Line Items]
Interest rate decrease								0.25%
Term Loan Facility [Member] \| Fourth Amendment \| Minimum [Member]
Debt Instrument [Line Items]
Interest rate decrease								0.50%
Original Revolver [Member]
Debt Instrument [Line Items]
Revolver interest rate					2.00%
Percentage over variable rate		3.00%			3.00%		3.00%
Outstanding letters of credit		0			0
Available borrowing capacity		240.8			240.8
Unused commitment, percentage					0.50%
Debt instrument, maturity date					2017-07-31
New Term Loan Facility [Member] \| Fifth Amendment [Member]
Debt Instrument [Line Items]
Debt issuance cost										1.5
Loss on extinguishment of debt										(1.9)
New Term Loan Facility [Member] \| Fifth Amendment [Member] \| Third Party Costs Cash Expense [Member]
Debt Instrument [Line Items]
Loss on extinguishment of debt										(1.0)
New Term Loan Facility [Member] \| Fifth Amendment [Member] \| Unamortized Debt Issuance Costs And Discount On Debt [Member]
Debt Instrument [Line Items]
Loss on extinguishment of debt										(0.9)
Senior Unsecured Notes Due in 2023 [Member]
Debt Instrument [Line Items]
Debt instrument, face amount											700.0			730.0
Debt instrument, interest rate														6.00%
Debt instrument, maturity date					Apr. 01, 2023
Debt obligations		1,430.0			1,430.0
Debt Instrument Issuance At Premium Price Percentage					1.00%
Proceeds from debt					1,437.3
Accretion of premium on debt					7.3
Senior Unsecured Notes Due in 2023 [Member] \| Debt redemption in whole before April 1, 2018 [Member]
Debt Instrument [Line Items]
Debt instrument, redemption price, percentage											100.00%
Senior Unsecured Notes Due in 2023 [Member] \| Debt redemption in part before April 1, 2018 [Member]
Debt Instrument [Line Items]
Redemption price as percentage											40.00%
Debt instrument, redemption price, percentage											106.00%
Senior Secured Notes due 2020 [Member]
Debt Instrument [Line Items]
Redemption of notes														675.0
Redemption premium		38.8			38.8
Debt instrument, interest rate					8.125%
Redemption price as percentage					105.75%
Standby Letter Of Credit [Member]
Debt Instrument [Line Items]
Outstanding letters of credit		$ 9.2			$ 9.2
Unused commitment, percentage					0.25%